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.