Steve -

I had the same problem and was able to correct it by using a center punch on
the pins. Pound (lightly) the pin nearest to the outside corner of the square
to correct a square where the angle (inside) is more than 90 degrees. If the
inside angle is less than 90 pound on the pin nearest the inside corner. I had
to take more than a few whacks at it, checking using your method of flipping
the square after each set of poundings, but it corrected the square quite
nicely.

Of interest: last weekend I visited the Eagle Square plant (Stanley) in
Shaftsbury, VT and watched a demonstration of (among other things) truing up a
rafter square. Essentially the same idea, but because it's one piece of flat
metal and there are no pins a machinsts hammer is used to pound and either
expand the metal near the square's outside corner or near the inside corner,
depending on the direction of alteration required. The operator keeps the
hammer going all the time bouncing it on an anvil with a steady rythm when he's
not actually tapping the square, and being ultra careful to have the hammer
head hit true so no dings appear on the square. 

Cheers,

John
JN in NH