Diana Model-25 - 1926.
With thanks for these pics to Keld. It appears to be a very early example of a Diana 25, dated September 1926. It has both the Diana Luft-Gewehr barrel stamp and an early version of the Diana goddess logo, but no model number.
Keld says it measures 97.5cm and the earliest Diana 25s I can find listed in Diana literature are said to be 97cm, so the length is consistent with it being a mod 25.