Thanks for these detailed replies, I really do appreciate them.
The "number" on the worn piece turned out to be a bit of lint. Sorry. No number there, but there does seem to be an "H" on one of the other pieces.
As for those other 5 pieces, here is a front and back photo of one. All these pieces appear to have a triangular "hole" on the bottom side where they broke off.
I haven't checked, but as far as I know at the moment this is a '74 915.
