I am gonna give a slightly different answer:
Are there bad 12M's ?
Arguably, yes, just as there are lemon cars.
Are there bad 'years' to be avoided ?
No, there are not.
Because the fact is, the design of 12M's (and ALL Artist series models, really) remained the same from the 30's into the 70's (or in the case of the 12M, into the 90's actually as UMI kept making these models after they took over Conn). I casn take a 1969 neck and slap in on a 1939 12M and it will perform just as well. I have taken keys off of a late one to use on an early one and they fit like a glove, only the minor-est of adjusting needed. It's the same design. (I might even argue that, in the case of ALL Artist horns, the same design was kept for a little too long).
But, essentially you get the same horn. I will (respectfully) not abide by the notion that nickelplate keys = the point of the downturn. I have played dog Artist horns from the early 40's, and killer Artist horns from the late 60's.
The pre-1962 ones are more sought after because the have the Lady engraving, and the brass lacq keys are more sought after because folks prefer brass lacq keys. The older they get, generally, the higher their market values. But the design and mechanics never really changed substantially. (I do wonder when the Fork Eb disappeared, actually ?)
One CAN claim that the N-serialed ones, Mexico-made, had some precision issues...and this would be accurate. It's the same design and a '57, etc...but the precision of fabrication sometimes tended to be inconsistent. But those issues are also easily correctable by a tech, and once corrected, they hold. So I have in fact sold Mexico 12M's to folks who could not afford a USA one, their market values literally being half of most USA ones....and they were very happy with their horns.