Like your holster, the answer is in two parts :-).
First part: the holster itself is by J.M. Bucheimer and is called a 'semi-shoulder holster'. It was a 'thing' in the 1960s.
jm bucheimer (48).jpg
Second part: missing is the paddle that attaches to the backside. Assembled it looks like this:
jm bucheimer (49).jpg
Separately the set looks like this:
jm bucheimer (50).jpg
I have seen the paddle alone for sale on an auction site (having had to tell the seller what it is) and it appears from time to time in my routine search there for vintage holsters; but I'm unable to find it now to PM the info to you. Perhaps I'll remember your quest if/when I see it again :-).
There were many makers of this style and I wouldn't expect their paddles to interchange (tho it's possible) so likely you'll want to be on the lookout for a piece from a J.M. Bucheimer only.