I'm not surprised that the low pitch is too low. As I (and some others) have observed in other threads and other forums, neither "low pitch" nor "high pitch" had unique meanings in that period. My Buescher is designated as "LP" on the second valve and was pitched to 435 prior to my cutting it down.
After being certain that the instrument is really clean (!!!), doesn't leak, and that you have a "reasonable" mouthpiece, then -- given your desire not to modify its existing parts -- I'd have someone craft a new tuning slide for you that should allow you to play to 440. I think that something like a leadpipe extension will not work out very well. My guess is that it would yield poor intonation in at least some places. But you can try this with some flexible plastic tubing and duct tape . That may also give you an idea of how much length would need to be added to the (high) tuning slide -- though your repair guy will handle that all by himself.
I'm more than a little skeptical about what the intonation will be like no matter what you do. It's a bit tricky to manufacture a valved instrument so that you can get the valve tuning circuits sharp enough to provide good tuning/intonation for the high tuning and also get the tuning circuits flat enough so you can achieve good tuning/intonation for the low tuning. This is ESPECIALLY tricky for the second valve on a baritone/euphonium where there's not much length to work with at all. So any fingering involving the second valve may well turn out to be wonky. My Eb horn still suffers from that a bit, and even on the Eb tuba there's almost NO room to do anything about it. Consequently, even if you get it so you can "tune" to A=440, the overall intonation may be pretty unfortunate. Don't expect too much. I've come to suspect that a lot of the nostalgia about these old horns made in the WWI and post-war period -- many models of which seem to have been made with a "town band" market in mind -- is mostly just nostalgia and bears little relation to how they actually played.
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)