US Model 1916 Dismounted Pistol Holster for the Model 1911A-1 pistols. As others have mentioned, markings probably indicate Bolen production with the part number/drawing number/federal stock number/contract number. Time period of production is probably mid-1960's to early-1970's.
The US Army transitioned to all-black leather gear in the mid-to-late 1950's. All holsters remaining in unit inventories were dyed black, usually leaving the back sides undyed to avoid causing stains to the uniforms in use. The popular term of the era referred to the differences between the old "brown shoe Army" and the new modern Army.
The last M1911A-1 .45 pistols were received in late 1945. No more were ordered or accepted after WW2 other than a few special purpose pieces such as National Match guns. No new holsters or other accessories were contracted until the 1960's when combat units were deployed to Vietnam, and many of those remaining in depot storage were found to be afflicted with mold or mildew. New contracts were given to Bolen, Bianchi, and a couple of other companies for M1916 hip holsters and M3-style shoulder holsters (tankers, aviation units, etc).
While serving in Vietnam I carried a M1911A-1 made by Remington-Rand in 1944 using a M1916 holster produced by Sears Saddlery in 1943, dyed black and stamped "MRT 1960" indicating Mildew Resistant Treatment applied in 1960.