When I bought my P226 it came with 2 "Italian" mags, 15 rounders.
Since then all my other mags purchased have been MecGar 18 rounders, bought as MecGar from either Dillon Precision or Top Gun Supply. These are great quality mags and cost significantly less simply because it doesn't have Sig stamped on them, same mag though. No FTF's.
I have no experience with the Checkmate's but sounds like I don't want any. I handled a couple of their 1911 mags, made a rude noise and stayed with my Wilson Combat 47D, way better construction. JMHO
If I have to explain it, you probably won't understand it.