Here's a description from TheHornGuys webshop of the plug-in 5th valve Dave mentioned. It includes information on tuning and fingering patterns.
https://www.hornguys.com/products/st...-321-euphonium
While the 4th valve slide design is different between the 2280 and 321, I would imagine you could get a similar plug-in valve designed and created to fit the 2280, if this is the route you decide to go down!
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)