The Buescher horns with the "Micromaster" valves, mostly 400 and Lightweight 400 models from 1937 through 1955, use a valve spring that is no longer common in the US.
However, my research implies that a common spring in the UK and EU, a size allegedly used by various Bessons, is the same size. It appears that it might be your 12.5mm OD by 40.5mm length.
Before I try to source them, can you guys using those springs check the inside diameter for me? Mine measure 0.460", which is about 11.7mm. If you have a medium-large bore horn, it'll more or less slip snugly over your tuning slide leg, or just barely won't.
I really appreciate your time, and thank you very much!