Ok, I may have mispoken. I just realized you said low C, rather than D# -- which for some reason is what my head centered on.
My 139 is out for restoration at the moment and I'm not expecting it back for months, but I'll measure my Big B (129) tomorrow if that will help you. The 139 is a very different horn dimensionally than the 129 (TT and Big B 'crats), so I seriously doubt they're the same size on the C key either.
I looked at saxpics.com to see if the 2 shown there had the same crook brace. Hard to tell for sure, but I seem to remember my previous 139 had a different crook brace than yours. That said, given the pics on saxpics, I could be hallucinating and yours is what should have been on the one I have with Matt Stohrer at the moment.
These are pretty rare horns. I'd be interested to know how you came by yours?