TromboneChat

I either can't search with correct terms or am a bit thick...but I can't seem to find other Mt Vernon mouthpieces like this one? What do the extra stamps mean?

It has the number 11 on the cup, a letter F on the throat, and the number 413 on the shank.

Thanks, and hello! This is my first post, have been reading the boards thinking about purchasing and Bach 36bo, lots of a helpful information on here.

Sounds like it's an 11 (rim & cup) with an F throat (.257) and backbore #413.
Bach backbore numbers don't mean anything except their own tool number for the reamer. I don't remember but I think 413 was a standard backbore for some of their mouthpieces.

I bet Doug is right.
This sounds like a special-order smaller-cup version of a Bach 6½AM [Cup I.D. = 1.00"; Throat = 0.257" (F); Backbore #413].
Cup I.D of a standard Bach 11 is nominally 0.972" [but I have one that measures 0.992"! No one ever said that Bach was consistent.]

Thanks!

"F" would seem to b a standard index drill bit size.

But considerably larger than standard size for an 11