This strange creature turned out to be a Mellophonium - which is a kind-of adaptation of a French Horn, but with three valves - it was bigly featured by the famous Stan Kenton band (one version of which was actually named the Stan Kenton Mellophonium Orchestra) - BTW this is some really superb jazz soloing:
And another mellophonium player recorded in a band with the famous alto sax player (of Take Five/ Brubeck quartet fame) Paul Desmond:
Anyway, it was clearly a remarkable instrument (despite being out of tune with itself). It had a niche filling the gap between trumpets and trombones, could play very rapidly across a large range, and with a wide range of tonal expression from smoothly-mellow to brazen-brassy.
But it never really settled down in terms of shape and construction, it was difficult to master - and for whatever reason it became all-but forgotten; and passed me by until just now.