Interesting point about MTOs and T2 mounts. So far I tried M42 and Nikon T2 and it works.
I just measured thickness of T2 adapters I have here and:
M42 is 9.4mm, Nikon is 8.6mm and one with out marking is 11.4mm thick (Minolta MD?? Konica??)
Don't forget that MTOs also have focusing past infinity mark so you can try simple thin Nikon to m42 adapter with out correction lens, (one with lens are mostly horrible) and check how far away you can focus with it.