Using the MW6502 as an example, which is used in a variety of Monitor models such as the 4, 4.5, 5, 5jr, 7, etc. (but this could apply to any driver that's used in several models)
The label on the back of the driver shows a list of models it was "Engineered for use in" with little check boxes next to them, and the appropriate model may be marked in some way and usually a big red or black stamp of the model is also stamped on the label (such as 5B).
The image below is an MW-6502 apparently from a Monitor 4 as indicated by the stamped M4
Is there a visible or design (either in the magnet, frame, voice coil, etc), difference between an MW6502 marked for a 5B, vs one marked for a 4 or 7, etc??
I suspect not, but then why differentiate on the label or mark it. It specifically says "Engineered for use in:" so one might presume that since there is a choice, that the design might be different. Kind of like a packaging box that shows two color options (Black or Cherry) with a check box next to each.
Only other reason I can think of is for warranty reasons such as to prevent switching out bad drivers from an out of warranty speaker to a speaker that's in warranty that has the same drivers.
I haven't had the chance yet, but there is a visible difference in the dust cap between a pair of Mon 5B and Mon 5A (or something like that) that I have. Also there IS a difference between the vintage MW6509 driver and the newer MW6509s currently being produced, most apparent is the doped paper cone and rubber surround are distinctively different, its stiffer (not as loose) and I've seen a subjective report of it not sounding the same (which could just require more break-in time) - But the visible difference is there none-the-less but the MN is the same.
I am mainly asking to be sure that if I buy a vintage driver to repair one of my speakers, do I need to concern myself with what model speaker it came from , or as long as the MW# is the same that I'll be good to go.