When is a 67 a 6x7 ?

Have purchased a (supposedly) 67 on auction. On receiving camera (body & prism only) I find 67 marked on top plate as normal,but no MLU switch on LH side of mirror box (when viewed from lens mount position),my suspicion is that it is an early model 6x7 which has been given a 67 top.
Am I correct in 1) thinking a 67 should have a MLU switch.
and 2) A 67 has a polycarbonate mirror box and not a metal one. Please help if poss.
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I don't think you could put a 67 top plate on a 6x7 camera. The 6x7 has two main versions, originally without MLU but later with MLU and much preferred.

The 67 is still a sturdy camera but has a lot more plastic in its construction and is also in possession of MLU.