It is either a M-3 or M-7 originally designed for air crew and tank crews in the WWII era. This one is for a 1911 ,45 Auto. The difference between the M-3 and M-7 is the harness, and I can't see enough of the harness to be more specific. There is also a model for the 4" Victory Model revolver. Here is a link to Pacific Canvas & Leather, the page specific to these holsters. PC&L makes excelent reproductions of many WWII period holsters from several countries and for several gun makes/models.
U.S. – Pacific Canvas and Leather