Puzzling, indeed.
First this lens/shutter combination: The lens number indicates 1919 or 1920 as its year of production. Could this shutter ( an Ilex #3 ) be from the same time? AFAIK Zeiss mounted its Protars in German-made shutters - in Compurs or Compounds by Deckel.
Second, how come that such a strange beast of a lens went unnoticed by major authors of photographic books of that time? I am pretty sure that I have not seen a 1:21 Protar series listed in the Zeiss literature collected at Deutsches Museum library.
Third, what should have been the purpose to build a Protar that is almost two stops slower than the common kind ( 1: 12.5 Protarlinsen series VII, VIIa), and still approx.1/3 stop slower than wide angle Protars (1:18) ?
I am not certain whether all Zeiss Protars had a patent number engraved on the lens mount , but the ones I owned had this reference engraved. Didn't Zeiss feel a need to distinguish such a special lens by getting an additional patent for it and by marking this visibly?