Shamus, welcome to NitroExpress.
The 280 Halger rifles were built up by Halbe and Gerlich (Hal-Ger) between the wars.
I think there were fewer than 200 built up. I owned #120 for a while and my brother had #105 as well. The numbers on ours were engraved on the top of the action wall.
They used magnum length Mauser actions.
Word has it that the 280 Halger case is derived from the 280 Ross but I would have to measure both of them up to see for sure.
Please post some pictures if you can.