It is simply a brass stud as used on flap holsters since

by Rob Leahy , Prescott, Arizona, Tuesday, March 10, 2015, 10:28 (3486 days ago) @ stonewalrus

before the 1860 until the 1917 Half flap that I was issued in the early 1980s.
You can easily acquire them via Tandy Leather, and add them as one would add a rivet.
Really the big brass hard to find mushroom shaped head is a bit better, but not much...

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Of the Troops & For the Troops