You're so correct, Pat S; the science behind all this eludes me. I was so skeptical about the use of bracing (or not) affecting the tone and playing characteristics of a horn. This was a perfect test of that theory since, as I mentioned, I was playing the horn with no knowledge of anything being changed.
Patkins, I'm just so thankful I called the shop and told them to find the missing brace. Doubly thankful that they found it. How it was missed, nobody seems to know.