Orthophonic Sound Box - tiny letter on back-plate ?

[De Soto Frank]

I have noticed on several of the pot-metal Orthophonic Soundboxes in my collection, a small letter scratched or stamped into the back-plate, usually under the letter " O " in the word "Orthophonic", and above the patent info around the mounting hub.

So far I have seen the letter " B ", and the letter " C ".

One OSB is the original one to my Great-Grandmother's VE 8-12X, another is from a portable ( early pot-metal, with phase-plug, and slots in the spanner ring.). Believe the third example is from an "indoor" machine.


Has anyone else found these "ghost" letters on the backs of OSBs ?

Does anyone know what they signify ?


Thanks !