Anything that I can find on Seitz is pretty sketchy, but here is what I have:
Seitz & Cie. AG (founded in 1850) was located in Kreuzlingen (Emmishofen before 1928), Switzerland (near the German border). After 100 years in existence, the firm merged with MOWAG AG, Kreuzlingen (founded in 1946), in 1951, to become MOWAG Seitz & Ruf AG, which, in 1954, became MOWAG Motorwagenfabrik AG. The company is currently known as General Dynamics European Land Systems - Mowag GmbH.
I haven't been able to locate any information on the puzzle car, but I did determine that Seitz had produced a number of electric drive vehicles, so I'm guessing that this vehicle may be an electric drive, although I understand that Seitz produced some serial hybrids, so this might have been an electric drive with a diesel (Seitz made an under floor model), or gasoline, engine to drive a generator.
As for a date, I can only speculate, based on cab style, that it is from the 1940's.