Sorry to come late to this thread.
You need to determine what the horn is pitched to. Is it A=440 or is it higher pitch A>440 or lower pitch A<440? This will determine how usable the horn is and whether it might reasonably be converted to modern (A=440) pitch or not: Low pitch=yes, High pitch=no, without a lot of pain and $$$.
Your selection of the Kelly 25 is an interesting choice. After a LOT of experimentation, it is what I've chosen for the best mouthpiece for my old Buescher 1924 Eb horn. But I also cut that from A=435 to A=440. I think you would find the Kelly XS to be good only if you want to play in the staff and above. But that's just a guess based on a large number of bass trombone and contrabass trombone mouthpieces I tried.
Re-hab: What needs to be rehabbed? I removed a large number of dents from mine and polished the stuffing out of it. The plating on those horns is generally thicker than on ones today. Mine obviously had the "double plating" option offered by Buescher at the time. Yours may need new water key cork(s) (use Valentino), maybe new springs? And CLEAN (and clean and clean and clean ...) it inside. Flush repeatedly with soap and water. Use a trombone snake on it. Do it until your arms fall off -- or at least until you get totally clean water when you pour it out.
Does that Kelly actually fit into the receiver? The receiver on mine is the "small shank" -- about (but not exactly) the same size as a bass trombone shank. If it's that, you might want to replace it with a standard shank receiver. Or sand down the shank on the Kelly to go further into the receiver. I believe that I have the last adapter (small-to-standard shank) that Matt Waters made for these, and I'm hanging onto it. But at some point, I may just pull the old receiver off and put a standard one on.
Intonation can be wonky. Well, intonation WILL be wonky. Mine's quite good now, but that's after cutting it down to A=440 and tinkering a lot with the receiver, use of a sousaphone bit in it, and the Kelly 25. It has good "ghost" tones and so can play quite passably in the contrabass register all the way down to the EEb fundamental.
Message me, if you want, about other questions.
Last edited by ghmerrill; 07-27-2021 at 10:15 PM.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)