I found this square, burnt and broken, in an old school which I decided to fix, and true its measurement. I began by taking the T apart and using my sledge, flattening the surface which would rest upon whatever edge I was squaring against.
I don’t have a fence on the table saw and the longer of the two pieces is tapered anyway, which meant I had to use a block plain to make long passes until any notches were removed. This did a pretty good job at flattening the edge, which would eventually allow me to draw against or score using a glass cutting tool. Following that, I cut this piece shorter using the chop saw while keeping the edge I planed against the saws fence to remove any cracked and damaged wood.
Screwing these back together should be done slowly, while taking constant measurements with the longest engineers square available. Had this been a normal carpenters square you could have flipped the piece over to compare the lines drawn. I finished by using a digital bevel box to test the how square I had secured the two pieces.
When working with exotic hard woods its a good idea to wear a dust mask as many are carcinogenic.