I pestered the seller. Hopefully he will add a bell engraving photo at some point.
It is a "compensator" which my impression from staring at a beaten up old BBb version, is what you might get trying to implement a blaikley system with the pistons from an enharmonic tuba.
"In this world you have to be oh so clever or oh so pleasant.
For years i was clever, and i recommend pleasant"
-Elwood P Dowd.