Martin's last pro horn was a variation on the Committee with extra engraving and silver plated keys/rods, similar to the Centennial.

The Magna is significant because it introduced a beautifully designed low A bari -- still missing the chromatic F# key, though -- that has a uniquely designed low A mechanism that can be operated by either the right or left hand thumb.

The G# cluster is somewhat redesigned on these horns, with a roller standing in for a full key for the low Bb. IMHO, this setup is far inferior to the standard Committee models in this regard, and is probably the least ergonomic feature since the Master model.

I have played with both a Committee and Magna tenor and I can't tell any other significant differences between the two. It's possible that the bore sizes are slightly different (definitely the case for the low A baritone), but I doubt that there is anything as significant as this.

Please note that some models were available with sterling silver necks.