The biggest change to the Conn 10M occurred around 359xxx (1955), when the neck was changed from the old style neck to a double socket and nickel plated keywork was introduced.  The tonal characteristics changed markedly.  The earlier horns are deeper and richer.  The later horns are brighter and more spread.  I personally was not satisfied with the sound of the later horns.  They're essentially Directors with upgraded keywork.  Some feel that the later horns are more mouthpiece-friendly than the earlier ones.  The later ones reportedly get along with smaller chamber setups better than the earlier ones do.  1947 - 1954 horns have  the old style neck and straight tone holes.  They sound very similar to the rolled tone hole horns.  The alternate eb key is on the rolled tone hole horns, but not on the later horns.   

My horn is a 1951.  The workmanship does not impress me in the same way the workmanship on earlier Conns does - some of the key alignments aren't quite as true and a subtle change to the LH table makes adjustment of the G# articulation tab more problematic than on the earlier 10Ms.  But those issues are not a deal breaker for me and the 1947-54 horns are much less expensive than the rolled tone hole ones.  

Rolled tone holes are nice, but if they're out of level they are a much bigger problem than straight tone holes.  Leveling rolled tone holes is a job for a Conn specialist.  A garden-variety tech can do some serious damage.  The chimneys on rolled tone hole horns are thinned by the process of rolling the rims.  Figure big $ if a rolled tone hole horn ever needs that kind of work.  

10M keywork isn't bad by vintage horn standards.  I don't have large hands, and I get around just fine.  The LH table does require a little extra work.  If the keywork of a 10M is a deal breaker for you, a Keilwerth SX-90R is worth considering.  It's the only modern horn I've played that comes close to the 10M sound.