Does anyone know what kind of pick this is?
General mountain dulcimer or music discussions
The shape looks like the large triangle pick sold by Fender and D'Andrea.
Pick Punch sells a make-your-own-picks punch in that #355 shape. It is very possible that you have a "home made" pick.
For the punch, see
Pick Punch sells several other shapes. Locally we have a couple of #351 punches. They are good tools.