I think it is pretty well established (mostly by lots of other threads here with "impossible" early serial numbers with late Voigtlander series) that the missing serial numbers were allocated to Voigtlander USA and used by them in, at present, unknown way. The idea about use on prototypes has no evidence to support it - in fact, most industry prototypes do not have serial numbers or even "commercial standard" engravings.

I think most, if not all, USA "reserved serial number" lenses have USA clearly marked on them and all engraving work is done in a slightly simplied script.

Yours is definitely from Germany. it is 6xx,xxx rather than 6x,xxx! It is from 1931 approximately. The shutter is earlier, but remember that the enormous recession in the late 20's may have meant that shutter stocks from earlier years may have been used in 1931.