Iwasaki's book has an interesting section showing close up photographs of guppy gonopodiums. One of the ways to distinguish livebearer species from one another is by their gonopodiums.

In most cases the elaborate arrangement hooks and other appendages must fit with the female's genital opening in order for effective mating and fertilization to take place. I shared a site a while back that had photos of guppy and Endler's gonopodia, which to this amateur, sure looked similar. (John Dawes' book on livebearers also shows a number of gonopodium photos for different species.)

Noburu Iwasaki in his Guppies: Fancy Strains and How To Produce Them speaks a lot of Japanese guppies and foreign guppies. A cursory look at his text suggests that he is referring to homegrown Japanese guppies and imports, largely from Singapore, sometimes from Germany. He criticizes Singapore guppies for the curse of the first week - often caused by medicine or salt put in the water. (He also offers ways to deal with that.)

One of the astonishing things in his book is a comparison of local and Singapore guppy
gonopodia. Those of the foreign and domestic strains are often a little different from one another. The Japanese guppies have a one hook or clasping mechanism. The imported guppies may have a clasper and a locking mechanism which is rather different! (p127)

They diverge much more significantly than the gonopodia in the guppy/Endler's series. So inbred might some domestic strain become, that key reproductive characteristics are becoming different!

They are still the same species. But such divergence of reproductive organs is usually associated with speciation!