I am a bit confused.
The opemus 6 came with three heads. A lamp housing with condenser etc. or a color 3 diffuser head and a color 4 (from memory). I have one lamp housing and one color 3 head. They are interchangeable.
The color 3 head sits on various enlargers and with the right diffuser box can go up to 6x9 there is no condenser. This is from the manual.
I think you might be right that it doesn't go past to 6x6 but there is some crossover with parts as i have a sub box in my full set that is stamped 6x7.
I wasn't very clear on my first post but there is a strong likelihood you actually have a 6x7 color head set up which could be as simple as looking for a larger b&w meopta enlarger to pop the color head on. Possibly even some broken ones.
As i say mostly the above is from memory so i could be wrong but i did have some headscratching when i ended up with two enlargers to work out what was what.