Kramers holsters are molded to fit specific models...thats why they fit so well. 1911's with rails, or some with full length dust covers...will need different models of Kramer holsters...although I carry a variety of more standard 4" or 5" 1911's in one of two standard 1911 holsters I have, one IWB and one scabbard. It depends which 1911 you have ...like a Sig 1911 will not fit a standard 1911 Kramer holster..
They are also designed to stay "open" so you can reinsert a gun in the holster one handed...so it doesn't collapse on you after you draw the gun. That's a really big deal...