Quasi-calibrating an old Hitachi-V223

[microbug]

I recently bought a Hitachi V-223 analog scope on eBay (£15 / $24, can you believe it?!), and after some cleaning up (contact cleaner on pots, alcohol on control panel) it seems to be working fine, though the horizontal gain is out of spec.
As you can see from the pictures, I found the horizontal gain trimpot and I can tweak it to get either its internal reference square wave (.5v pk-pk) or an AD9850 module also from eBay to be spot on -but they do not match! If I get it accurate for the sine wave from the module (tested both @ 1MHz and 1KHz), it isn't accurate for the square wave. This isn't because they are two different ranges because the square wave is 1KHz and if I test the sine wave at the same frequency it is still out of cal.
So which should I trust? I would place my bet on the module based on both what I've heard here and on this blog, but I'm a noob! - what is your opinion?

[sync]

Don't trust the probe calibration signal. It's usually not build to have an accurate frequency and amplitude.

[microbug]

OK - glad to see I was right