Unknown microfinish symbol

I have a drawing of a piston for a hydraulic cylinder drawn in 1968. The symbol in question is a square sitting on one of its corners on a horizontal extension line for the diameter of the piston. The line from 12 o'clock toward the 9 o'clock position does not quite close the square, and there is a "15" inside the square. The supplier quoting the part asked for an explanation of the symbol, but nowhere in any of our reference material, SAE, Machinery Handbook, ANSI or ASME does that symbol appear. One might assume that the microinch is 15, but is there something special about that symbol that it was used instead of the more traditional "check mark" symbol?

Hello David. I've never seen such a mark either and I don't think it has any accepted meaning. My first guess is that there was once a list of notes, and Note 15 applied to this dimension or surface. My alternate guess would be that it is meant to indicate a 15 degree chamfer because there does not seem to be any other dimensions or notes addressing what appears to be a chamfer unless the .015R is pointing to this in the cutoff side view.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey