The Colt Army Special, Official Police, Officers Models, 3-5-7, Python, and Trooper revolvers all used essentially the same medium frame, the main difference among them being that those chambered in .357 have the firing pin in the frame instead of on the hammer. They are a little beefier than the S&W K-frames. The Colt New Service revolver and its derivatives used a larger frame, somewhat equivalent in size to the S&W N-frame. The Colt Police Positive, Police Positive Special, and Detective Special (and its derivatives) revolvers used smaller frames. I'd think most holsters made for the S&W N-frame should accommodate a Colt New Service and vice-versa. Military holsters for the M1917 certainly will.