VM:
There is not a lot known about these rather complicated holsters. As mentioned they apparently were intended for use with the Model 39. That has led to speculation that these are Navy SEAL holsters, although as far as I know no direct connection to the SEALs has ever been firmly established. An NSN for this holster, 1095-00-804-7725, was established back in 1968.
Scott Meadows described this holster in his famous "US Military Holsters" book, but was unable to offer much in the way of detail on it.
I have a couple of examples in my own collection. One in new in the wrapper. The label describes it, rather curiously, as "Holster, Revolver, S&W". Of course, no S&W revolver will fit comfortably in this holster. Below is an image of another example of this holster (not mine) with the packaging I describe.
Another anomaly is the CAGE code which is supposed to identify the manufacturer. On my packaging the CAGE code comes back to some obscure aircraft electronics company, not an outfit that I would expect to be in the gun or leathergoods business.
That is about all I can tell you. I hope it has been helpful to you.