It looks like the 297/250 was developed by H&H around 1880 or slightly earlier. I expect that for training purposes, the .207/250 was superseded by the .310 cadet which was developed by Greener around 1900. Assuming that other rifle manufacturers started using this cartridge sometime after H&H released it, the rifle could have been built as early as 1885 +/-. Likely, as a training cartridge, it would not have been built much after the .310 was released so say not later than 1905 +/-. Of course, those are merely WAGs, based upon limited facts.