Yes, Rock Island Arsenal. The were made to hold a Victory revolver. Yours looks like it was originally brown, which would pre-date the mid-1950’s. The USAF were the first to change to black in 1950 and the other branches adopted the change a few years later. A number of older holster were dyed black to keep them in service.
I’ve seen later versions marked “MRT”, which stands for mildew resistant treatment. They were still used during the Vietnam war.
I’ve got a later one with a contract number made by Hunter Corporation. One of the machine rivets obviously failed and was replaced with a Chicago screw. I figure it’s worth around $40. Yours may bring more.
There are reproductions being offered with spurious markings that sell for around $25, but yours has brass hardware and enough wear to make me believe it’s original. I think mine is also original, because I find it hard to imagine that anyone would counterfeit a Hunter holster!