Ok, lemme see what we've got:
Commercially cut down 30 round curved mag to 20 rounds, commercially injected molded floorplate w/ non-standard mount to fit to a cut 30 round mag. Made in an era where 20 round mags were plentiful and easily available; the only possible advantage to the curved 20 round would've been the curve, but produced before the era of self-leveling followers, so very little advantage.
Or (cynic hat on):
Back-room shyster operation selling "pre-ban" 20 round mags by buying cheap post-ban LE only 30 rounders, chopping them to 20s and thus cutting off the LE/Gov't only bullshit. Sold in an era when 20 rounders commanded a high price, and no one complained there weren't makers' marks or they'd never seen that floorplate before. Take only cash, leave no trace.