Oh, well....I think this thread will get few views...because nobody is interested in any Conn Artist models (6M, 10M, 12M, 11M) which do not sport the naked lady.
I say this half in jest...but it's true. IF your thread had been entitled '6M Naked Lady'...you would get 4x the amount of replies.
Silly....because, having had many of both thru here....I have done side-by-side comparisons between early & middle model Naked Lady 6M's.
Guess what ?
With the exception of some minor key design reworking....they're the same horn.
Same design, same spec, same quality of construction. True that the keywork construction (from the point of view of say...how cleanly the spines are brazed to the keycups) might be finer an the earlier horn if one were inclined to look really closely.
But it's the same blueprint, and the horn was made in the same place, the Elkhart factory...all the way up until 1970.
They sound identical...which would make sense when you use the same body tube, bow, bellpiece, and neck design over a period of 20+ years.
Some claim the '60's horns to be made of a lighter gauge metal on the bodies...could be, I have to admit I didn't check that....but I have checked the tone against earlier ones and it's pure 6M.
I agree with Bruce...if the price is right (and a 60's 6M in very good condition and playing shape has a market value of around $700-800usd - adjust for the fact that in AU the classic vintage horns tend to cost more because there are fewer around, so I would say if it is $900usd or so it's still a decent deal)...they are great horns.
Intonation is not an issue as long as you team up a medium to large chamber mouthpiece with it.
"Technique"...well...I assume you mean ergonomics....is fine and is really a matter of taste; but if you are coming from a contemporary horn, you are going to have to woodshed with the 6M for a few weeks to get it under your fingers.
Of course, your reward for that is having an Alto which sounds so much better than a Yamaha, it's absurd....kinda like how your 11M sounds in comparison to a newer-style low A....