That's news to me.On a try square only the inside face is square. the brass plate is the reference plane and the inside edge of the blade is the only one square.
Unlike a framing square which is square on all sides.
For curiosity how did you check this square for square?
I pick those up at flea markets and garage sales any time I see them. They do get out of whack pretty easy over time and use. What I've done is to start with a framing square that you know is square. Set in the framing square and clamp both legs to the square until it is tight. You may need to tap it a bit. While clamped drill two holes on opposing corners the correct size for a compression rivet, brass if you can. The drill bit has to be the right size for an exact fit. place the catch portion of the rivet in the underside and tap in the other half with a flat drive pin. Make sure you back up the backside while tapping in to support the action. The rivet should be close to flat when done. I'm not always successful.
I don't know why I didn't think of that. I don't mean giving it to the shop, I can just square it up myself with a file. Or a drinding wheelHears a thought. Bring it buy you local machine shop and see what they would charge to hone it back strait. It caint hurt to ask.