I'm not sure if he uses this forum, but I think Steven Mead would be well worth reaching out to regarding this topic if you haven't already, as I believe has been consulting on the design of the Besson Sovereign/Prestige euphoniums for many years.
He is fairly active on Facebook, so it may be worth reaching out to him through there?
Willson 2900 TA-1 Euphonium - Denis Wick 4AM
Yamaha YSL-643 Trombone - Bob Reeves BrassArk 5G "Gladstone"
Yamaha YSL-8440 Trombone - Denis Wick 5BS
VMI 3301S BBb Tuba - Schilke Helleberg
Past:
York Preference 3067 Euphonium - Denis Wick 4AL
Benge 165F Trombone - Benge Marcellus
Wessex BR140 Baritone - Denis Wick 6BS
F.E. Olds Special Trombone (ca. 1941)