There are three things that could cause magazines to not drop free from a 1911:
- the trigger bow
- the grips screw escutcheons
- the mag release
I have several spare mag releases, three of them large button types. I switched/mixed/matched mag releases and the same mags that would not drop from the SR1911 dropped free from the other guns. In the case of the SR1911, the same mags would not drop no matter what mag release was installed.
I have more 1911 mags than I want to admit, since I started becoming a "mag collector" back in the day of 7 rd mags so my testing was exhaustive. So I'm not any wiser.
Conclusion: I've got a few mags that are not compatible with this particular SR1911. Just gotta keep them separate.