They're called "studs" and yes they're for the button holes. Sounds like you got a complete set, which I have yet to run across at the thrifts (the handful I've seen have been missing a stud).
That reminds me, I recently thrifted a tux shirt that fits me better than the old one I've had for years and have outgrown by a smidgen. Not really worthy of pics in this thread (Joseph Abboud) though it is pretty decently made of Egyptian cotton, fits perfectly, and was $5. I looked at tux shirts at the mall last month and it was a two-hour exercise in frustration.