Thanks Chris.
Interesting. I will investigate the tilt adjusting screw that you mentioned. I have the original manual for the IIc, but there is very little information in it. Nothing on adjusting the tilt, and surprisingly very little on setting the autofocus. It is basically useless.
As you mentioned, I diagnosed the problem to be the column leaning to far forward, possibly from over tightening. So I corrected it with shims. Not Leica shims, though they seem to work.
Images look square now, but it just seems so "un-Leica" to have not designed something in the mechanics to correct this.
I'll let you know if I find anything.