I can't tell for sure from the picture but it looks more like a model 27 to me. Is it a full size or 3/4 size machine? The 28 and 128 were 3/4 size and the 27/127 machines were full size. Yours appears to be missing the belt guard and bobbin winder assembly as well. If it is a 28 in a treadle that would be a really neat find, I think the 28s were mostly portable machines.
My understanding is the records for the German made machines from that period were lost or destroyed during WWII.
I have a German made model 15 I can't get an exact date for either.
Rodney