I do love print books, but it would be tough for a book to achieve the interactive functionality of Don's pop-up watermark tool at StampSmarter. Very nice Don....
Looking at the owner's or expert's mark on OP's stamp, it appears to me to be a "J" and a "B."
I am aware of the experts' mark site at
https://www.filatelia.fi/experts/index.html which has been mentioned in one or two other Forum threads. I looked somewhat quickly through the B's (surname) and did not see anything likely.
I would also be very interested in some similar site giving owners' marks. Anyone know of one? Many such marks would be unknown I'd bet, but there might someday be utility in something along these lines that showed these sorts of marks. Or at least a vehicle to partially satisfy curiosity....
Here's one that I picked up a couple years back on the verso of a rather pleasing US 397. Was the owner a big chess player as well as a stamp collector? Or perhaps s/he was named "Knight?" Does anyone else have a stamp from this owner? Or from our OP's "JB" or "BJ"?
