That firefly pistol is a double action mag fed and uses a forward mag like the broomhandle mauser posted by Spinalmath only not ugly. I'd make my own shortened mags, I'd just want the front magwell made for TPX mags.
If the front proportions make the magwell too short to lock the TPX mags, I could make full custom mags.
The movie version would be the best choice, IMO. The mags in the show had a ring on them:
The mags are shown well in this pic:
The TPX valve and reg should fit well in the body, IMO.