We used to have a thread here on the forum which was devoted to original chin straps. It along with others, were highlighted so that they were always first on the list of posts. Unfortunately, these are no longer first in line but hopefully, they are not lost. So, do a search here and see if you can locate them. I have looked at this strap......the leather looks old, as does the stitch repair. What I have noticed on brass originals is that the edges of the buckles are rounded and the buckles themselves are thinner/finer than the modern copies. The repros are thicker and cut square with sharp edges. The edges on your buckles do seem rounded but it is still a difficult call here just from photos.
I spent years getting the ones I have and it was a hard slog, I think it was 5 that I needed. I don't like this one for it's unused like state, by that i mean it's still rather right-angled all the way along it like it's never taken a hit or a bend.
That stitching also concerns me almost like it's a new joint.
I always looked for where the buckle had moved over the course of using it and would leave an indentation or marks.
I'd suggest any of your finds post them here as they are out there, particularly on ebay (unfortunately, not too fond of fleabay), just like 911car has just done, there are lots here that would enjoy helping you....me included.