I'm posting this review as a heads up to all my fellow 10M fans on some rather substandard workmanship and quality control in evidence on this particular example. The 10M is one of my all time favorite models, at least the pre-1955 ones with lacquered keys and the standard neck tenon. I realize that not everybody agrees, but I am of the opinion that the later versions have a less deep resonance and an overly spread sound. By "spread," I mean that the tonal center is weak and lacks focus, with upper harmonics not supporting the tonal center and resulting in kind of a blatty quality to the sound. You can actually hear the difference doing the pop test. The early straight tone hole ones with the standard neck tenon are worthy peers of the rolled tone hole ones in terms of sound quality. Unfortunately, the workmanship is not always up to snuff. This 1951 example was problematic in terms of alignment and clearances, which led to problems keeping it tight and in regulation, and eliminating key noise. It was not until I overhauled it that I realized the root causes of the issues I was having with the horn.
Inadequate clearance between the G# articulation tab and the table keytouches led to the C# key opening slightly when the bell keys were depressed.
An excessive gap between the table B and Bb keytouches led to difficulty going from B to Bb. The problem was remedied by lowering the Bb keytouch. Not as elegant looking as having all in the same plane, but it works better.
The stack Bb key was misaligned southwestward, impairing functionality of the Bis key and making cup leveling virtually impossible until the alignment was remedied. Given the effort it took to align that key, I am quite certain that was a factory defect.
The G# bridge arm did not clear the Bb arm properly. The factory "solution" was to warp the F# key so the arm cleared the Bb arm, and move the G# key southward so the bridge contacted a higher part of the cup. That led to recurring problems with leaks on the F# key.
The side Bb key did not open fully, resulting in an intonation problem. Modification of both the keyfoot and the keytouch were necessary to get full opening.
The Eb tonehole was severely off-level, convex in the direction of the bow, and thickness of the chimney is uneven. The factory "solution" was to warp the keycup in the same direction. The bow had never been bashed in, it was a factory defect.
Palm D did not open fully, causing an intonation problem, and was misaligned over the tone hole.
Linkage clearances were often inadequate for installation of sufficient material to counteract key noise.
Some tube-to-post fits were on the loose side. Fortunately, this did not occur with rod-mounted tubes, so it wasn't consequential to key play.
Some keycups remain imprecise in their alignment over the toneholes in a manner that cannot be remedied without re-soldering posts. That could be consequential for someone going for a bleeding edge setup with precisely-fit resonators.
The factory defects on this horn included some egregious bloopers that never would have left the factory under decent QC. As you may have guessed, these were very time-consuming problems to diagnose and fix. I am not sure what the context of these workmanship and QC issues is. Conn was doing defense contracts related to the Korean War and may have been in a temporary crunch, or Conn's long-term quality decline may have already been underway. If you are considering a Conn from the early 1950s I suggest you check it out thoroughly for manufacturing defects and plan your overhaul budget accordingly.
Consider the two star rating as tough love for a horn that would never have been worth what I put into it if it didn't have that great classic 10M sound.