As far as mags go, I believe you need the older 15 & 16 round CZ type mags if you want a more flush fit. Others may or may not work/lock in properly, and may not fit flush to the bottom of the mag well. As in you may be able to use a 17,18, or 19 round extended mag but it may not set flush against the mag well. You may find flush fit mags at
https://mec-gar.com/ or
https://gregcotellc.com/cart/index.php?main_page=site_map . The frame will be closer to a CZ style that utilizes a non ambi mag release, and has the CZ style mag release & trigger bar springs that are secured with a screw. The Tanfoglio "Custom 90" was a limited run of pistols in which they used leftover parts from the contracted production of the ITM AT84s pistol parts and just leftover parts in general to build them. At the same time they were rolling out the newer, replacement "Series 88" pistols with frames using a reversible mag release that has a much better trigger bar spring and mag release spring setup.
The ITM AT84s was a Swiss pistol built with parts from Tanfoglio but finished by ITM (Industrial Technology & Machines) in Solothurn, Switzerland. Some good info here,
https://www.thefirearmblog.com/blog/2017/02/21/itm-at84-ultra-rare-swiss-made-cz-75-pistol/ The AT84s came after the AT84. The AT84, I believe, was the first licensed copy of the CZ75. The AT84s was more of a CZ clone and is believed to be the grandfather of the Swiss Sphinx AT2000,
https://special-ops.org/sphinx-at-2000-an-improved-version-of-the-czechoslovakian-cz-75-pistol/ The ITM pistols are somewhat rare and vary wildly in value.
I definitely love your pistols style trigger guard but not a fan of the mag release/trigger bar spring setup. The fact that it doesn't have secondary importer marks is fantastic as far as value and aesthetics. Though the finish is pretty rough on yours, it should still be a great shooter. It's interesting that nothing is stamped at all on the frame next to the slide lock/release. That makes me think the frame might've been destined for an ITM AT84s build. I had a pistol very similar to yours that had no markings in the same place as yours. After I had it refinished, you could hold the frame up to a good light source at the right angle and see where the frame was pre etched to receive the ITM Solothurn Switzerland stampings for an AT84s build. Clean up the frame on that left side, oil it and hold it up to the light and try to see if those same etchings are there. I'm kinda curious.
Hope this helps.
