Phenmetrazine is itself a "designer drug"; as sort of explained in the article, it differs from amphetamine by an oxygen and a couple of carbons forming a morpholine ring including the amine moiety, and its designers hoped it would have some of amphetamine's effects (like weight loss) with less side effects. As with most designer phenethylamines, this turned out badly, though not as badly as e.g. Fen/Phen. (There have been a lot of "designer" phenethylamines; are you really sure that "derivatives [of phenmetrazine] have just started to appear on the grey market"? Are you sure PiHKAL doesn't have a few?)
Yes, actually, there are no phenmetrazine derivatives in PiHKAL, and yes one has just recently started to appear on the grey market in the last few months. 3-FPM. There is one other derivative, phendimetrazine, which is prescribed for some of the purposes phenmetrazine originally was. It is considered a prodrug for phenmetrazine though, so not really a unique thing in its own right.
Phenmetrazine was originally banned because it was widely abused, not really because of any health reasons above and beyond the likes of dex-amphetamine, so far as I can tell, rather than anything like Fen/Phen which causes heart valve mutations.
