http://christophfunk.dk/files/3714/2174/9050/Sauer_artikel_rev2.pdf

The frame would date to say 1890 then the client came to Kettner post 1912 & ordered a fluid steel tubeset to apparently replace a patter welded set? Then Kettner's serial nummer 739 was applied to frame & tubeset. Probably a Modell Nr. 3 Sauer & Sohn.

Cheers,

Raimey
rse