I compared your PE shot with the other one I linked. The boards look totally different. So i looked at more pictures and found one here--scroll down a little over 1/2 way to the B3 overhead view. The picture you posted looks like what I think is the RAM on the left side next to the power supply; the picture at the site I linked before (with the WO and AP number) looks like the sound ROM on the right side.
By any chance did you transfer the RAM or the Sound ROM into the XL-7? If so, that might explain why it was thicker and why the OS 2.0 didn't see and Sound ROMs.
Mystery solved?
In any case, I'd go with 128 voices + 64 voices, you might prefer the 3 ROMs in the XL-7, though--assuming it works.