Thanks, I am putting some feelers out. I might replace them with conventional screws if I can't find originals, although your idea is interesting as well, Joe.
I need three of those for an Alexander 163 BBb that I recently purchased. It's been a busy summer and I haven't gotten to trying to track them down yet. I, too, was planning to email Alexander to inquire about them. If you discover that Alexander does make them please let me know (or you happen upon a stash of them somewhere and can spare a few, I'd be interested in working out a deal).
Alexander does, as far as I can tell, make this TYPE of screw for some of their currently manufactured instruments, but I do not know if they currently make it in a tuba size or with the appropriate thread size.