Pre-ban 21 mags not seating

FNG here with a predicament. Picked up some old 13rd g21 mags for my new CRB, and the U notch versions do not seat far enough for the magazine catch to engage them. The square notch pre ban mags do not have this problem. However, the U notch mags will seat and engage the magazine catch with their floor plate removed. It's as if the magazine body is just a bit too short.

I've dug around here using the search function, and have not found this situation discussed. I'm thinking I may be out of luck with these U notch mags, although others seem to not have this issue when using them. The U notch mags also fail to seat fully with the MagExt installed.

Any ideas?

Revisting. There's definitely a bit of interference where magazine well meets floor/base plate of U notch magazine. If I put more force at the front of the magazine, I can get it to seat fully. The distance needed to ease insertion is extremely slight. There is a bit of mold flashing at the perimeter of the mag well. A wet sand might be in order.

Pup check out my post way above 3-20-17 if you haven't fixed it already. You need to trim the very front of the mag base plate. It will click is nice and u should be set.