There are actually three variants right now (and soon to be more when the 4th generation with interchangeable backstraps get here). There is the original 21SF with the picatinny rail and ambidextrous mag release, the Amibidextrous mag release version with the standard GLOCK rail, and the Version with GLOCK rail and a standard one-sided mag release.
Yes the Picatinny version will not fit most holsters, and the ambi version with standard GLOCK rail will not fit some regular holsters either. Don't know why exactly, but I had a hard time finding a kydex holster that worked when I first got mine. Ended up using the smaller holster that was made for the 30SF/29SF, which for some reason fit. The Regular rail/regular mag release version will fit every holster I've tried made for the original model 21.
