Well, it's a bit of a mystery... (all part of the charm)
The .25-20 WCF barrel has got a London adress on it which in Nigel Browns book dates it at 1893-1908. And the makers name is "Charles Osborne & Co" and not "Charles Osborne & Co Ltd.", which suggests a manufacturing date prior to 1895. This should date my rifle to 1893-1895. But the serial number on the .25-20 WCF barrel suggests it was made in 1919. There are only one proof mark and it's the Birmingham definitive proof mark used until 1925.
The flats on the action have got German proof marks and the same Birmingham definitive proof mark as the .25-20 WCF set. The two other sets just have German marks and "SS", which I now think is the makers mark from the Suhl barrel maker Stephan Schilling or less likely Sauer & Sohn, Suhl.
I have a Charles Osborne & Co Ltd. trade catalogue from 1905 in which my rifle appears.
1272 - Anson & Deeley Pattern Hammerless Ejector Rook Rifle. Double Bolt Action with Automatic Safety. Snap Fore-end with Horn Cap. Dest Make and Finish throughout. Finely Bored, Rifled and Shot. File-Cut Top Rib. 13
The catalogue also contains rifles from Marlin, Colt, Remington, Savage, Stevens and Winchester, and Osborne was the sole importer of Smith & Wesson Revolvers to the UK. Import from the US seems to be rather important to Osborne at the time. Perhaps my rifle was made for export to Germany or Sweden in .25-20 WCF. The original barrel does not appear to have been altered.
Prior to me the rifle was owned by a wealthy and important family near Gothenburg, Sweden. A city which has always had a lot of trade connections with the UK.
It's a little gem, but in a rather sorry state, so it needs a lot of care and attention...