Timeframe should be around 1895 untill 1905, that would be also the timeframe for the DRGM first issue and the extending of it.

Because there is nowhere any info about an patent, not in old literature, not in new, not in related articles i think there was never an patent issued.

It makes no sense to get an DRGM and Patent at the same time, so i think they have planned to patent it, but used the cheaper way of an DRGM.