I bought a similar rifle at Melbourne recently ( except it has a part octagonal barrel and is .22 rifled ). It has no markings apart from serial numbers. On the top of the barrel an area has been neatly (professionally?) stippled with a fine punch and a border engraved; I'm guessing this was an import in the 1920's when there was prejudice against German products following WW1. Sometimes they were left blank completely and importers, such as Milbro, stamped their own name and numbers on.
You could try looking at the Vintage Diana Forum, there's some info including old catalogues with details of different models.
Cheers
torrens