He pointed out that plumetail platies show their tails from birth - that jives with what you noticed. If there is a plumetail fry though, that again suggests that the female was hit by a plumetail (platy, swordtail, somebody) at the store.

I was surprised by Bill's observation that both the high-fin and the lyretail traits, as with the plumetail trait, are genetically dominant to to the gene for regular finage! So my guess that if your lyretail male effectively hit the female, as these fry grow up, as many as 50% should show that trait!

That is why people breeding swordtails will mate a regular male swordtail to a female lyretail.  Those swords should produce a batch where half the fry will develop lyretails

Ironically when I Googled for an image of a swordtail gonopodium I got

www.guppylog.com/story/2007/7/13/193155/289

That was an image of a guppy gonopodium originally in something from the University of Toronto and their now reorganized Departments of Cell & Systems Biology (CSB) and Ecology & Evolutionary Biology (EEB). I'm really lost on their site, but you probably can see their image if you Google swordtail gonopodium. (I know, does that make sense - you Google swordtail gonopodium and get a guppy gonopodium!)

A link that works from that discussion is
http://www.diewasserwelt.de/endlersguppy3.htm
which compares guppy and "Endler's guppy" gonopodia. Notice that such organs have hooks and lobes and narrow extensions and wide extensions. That business is "supposed" to allow only males of a certain species to mate with females of their species because the female ventral opening would only be configured to accept gonopodiums and sperm of their species or a near relative.

This in an article investigating gonopodia of different species and variations (!) within species.

http://faculty-staff.ou.edu/L/Randall.B.Langerhans-1/gonop.html

Either there or elsewhere, there are observations that the length of gonopodia will influence mating behavior. Males with long gonopodia are more effective at "sneak spawning" with females. Those species usually don't perform much of a (or any) mating dance or courting behavior. Males with shorter gonopodia risk serious injury (as did some of your male swordtails) in trying to sneak spawn with larger females. Those males may have already been successful in spawning with those females and the females violently declined to copulate again.

If a gonopodium is amputated half-way, as you did with your lyretail male (and while that was very resourceful of you, I'm pretty sure that others have tried this before) that removes a lot of the end of the organ ,anatomically designed to fit to and "hold" the female while sperm is transferred.

There is still a legitimate question as to whether your male could effectively mate with a female swordtail, though the chance of some sperm being swirled through the the water to the female may be a tiny bit more possible.

The sperm can be stored as you know, for several months. According to Axelrod & Wischnath's Swordtails and Platies (TFH 1991), that sperm is stored in the folds of the females oviducts. Those authors suggest that 5 to 9 batches of fry may be produced with Xiphophorus! They also cite studies of guppies where the sperm from more recent matings almost exclusively produces guppy fry and contrast that with experience with Xiphophorus. They conclude that fry descended from earlier mating will appear from time to time with swordtails, platies and variatus.

They also mention that unfertilized (virgin) swordtail females will release eggs from time to time. Eggs must be maturing on a regular basis. If they get too crowded, some are released. The females probably eat those eggs, returning the nourishment to their bodies. I have been especially coddling a young female Fundulopanchax gardneri P-82 and noticed when feeding her the other day, that she was releasing eggs and trying to eat the eggs and the food given her. (Busy! Busy! Busy!)

That is actually pretty important (we may see female egg layers in a community tank doing this more often). If the female can not release extra eggs, she may become egg bound, swell up and even
die. That female gardneri is getting pretty chunky and I may put her with a male P-82 briefly, just to release her excess eggs. And
that may be an issue of life or death!

"Spawning out" egglayers like that is certainly not my idea. Aquarists who isolate females to condition them for spawning have done that for years. In a community tank with schools of tetras, danios, barbs, rainbowfish and the like, that probably happens (often at first strong light in the morning) fairly often.

Take photos. Watch your fry. If some show lyretail features (probably the odds are that they would be a little less than 50% of the group) then you and the male swordtail accomplished your goals. :) You may see this developing in a month or two according to Allen.

If they don't show lyretail, here are some things to consider:

You didn't mention how big the aquarium was with the swordtails in in, nor did you mention what other fishes were in there. There is a possibility that your female has been dropping fry (starting with a small number) all along and the fry have been eaten.

By way of illustration, lacking an available 20-gallon tank (or larger) I squeezed about a dozen red velvet swordtail fry into a 10-gallon aquarium, graveled and furnished with a few Java ferns and Crypts. I have been taking smaller males out and dumping them in a living room community tank. I left the growing females, the
largest male and some youngsters (dropped a while ago) of indeterminate gender in there. I am quite sure that the female swordtails have been dropping for some time now.

A couple of weeks ago I saw fry there. Assuming that fry were not surviving because of a lack of shelter, a couple huge handfuls of hornwort were dropped in there and some black worms were squirted into the feeding (soap-less-clean-ex-mustard) jar. When a friend recently dropped by to pick up one of the males sexing out, he
noted that a few of the fry were surviving and thriving. (He and I both have that red velvet swordtail line, originally from Michiana Aquarium Society stalwart Matt Bielsky, and swap stock back and forth.) I only need four or five fry, not the 50 to 100 being produced monthly in that aquarium.

Some attempted Xiphophorus crosses and been unsuccessful or have produced hybrids that developed cancers - hence their use by medical researchers. Other crosses, assuming that the shape of gonopodiums within the genus Xiphophorus are more similar to each other than to those of other genus, between swordtail species have been successful in producing quite an amazing catalog of crosses. Please see http://xiphophorus.org/images.htm

There is a lot there I haven't searched for there. By the way, questions about collecting in Mexico come up from time to time. One wants to avoid arrest and do that legally. Please see the discussion of this at
http://xiphophorus.org/collecting.htm

Back to the Bill Allen article. He noted that (as with black x black angelfish or black x black Bettas) it may be in many cases that lyretail x lyretail (through artificial) insemination may often produce a lethal gene that will kill many of the fry at a certain time period. (Sort of like sickle-cell anemia where humans with one gene with a sickle and one regular gene have a considerable resistance to malaria, but sickle cell x sickle cell often will produce fatalities.)

Bill also notes that lyretails can be crossed with hi-fin platies (or swords). These crosses will have the fancy finage. The male's high-fin's gonopodium is normal and they can inseminate female lyretails, producing roughly 25% normal fry, 25% highfin fry, 25% lyretail fry and 25% fry with BOTH highfin and lyretail characteristics. And again (sigh) that last group's males will not be functional.

Because that April TFH is the ALA 2009 convention issue, they pack several great livebearer articles in there. I think that they are all by ALA members too. Either browse that issue at a big box book store or just plunk down $5 (6 Canada, something else in the UK) and you can read, re-read and take notes at home. I'm sure you can get more from that swordtail article than
what was mentioned here.