As I gather from the above, this piece started life as a German rifle circa 1890 or earlier, retailed by the B'macher signed on the barrel, to be later acquired, not unlikely as a war souvenir, by a Britt who brought it home. As was not unusual with such antiquated rifles in obscure calibers it was bored out smoooth for a suitable generally available cartidge, and proofed.
As a side note, a single shot rifle made in Ferlach in 1945, under British rule and by their order, was originaly chambered in 7x57R and so proofed, but was later converted to .22 Hornet in UK and re-proofed in Birmingham.
With kind regards,
Jani
