Such underlever hammer drillings with those small, egg-shaped backlocks are not as unusual as you think. They are featured in every German gun catalog, wholesalers as well as mail order houses, from about 1910 to 1930 and are encountered with many retailer's names engraved on the top ribs.
German proofhouses from 1893 to 1936 stamped only the bore/land diameter of a rifle barrel. The Suhl proofhouse used the mm dimensions in .1mm increments from 1912 on. The nearby Zella-Mehlis proofhouse started earlier, in 1911, but added below the bore diameter, the case length and the proof date, numbers for the month and last digits for the year, and a ledger number for that month from the start. Suhl only added case length and date from 1923 on, so your drilling was proofed in Suhl between 1912 and 1923, as no other marks are shown on your photos, unless there are marks not shown yet.
The small town Heinrichs bei = near Suhl was independent until 1936 when it was amalgamated into Suhl.
The Gewehrfabrik = gun factory (every small gunmaker with more than a half dozen men and some lathes and milling machines called his workshop a "Gewehrfabrik" then) Gustav Meffert there was first mentioned in 1888. They advertized as "makers of hunting and target arms, specialists for drillings, cape guns, garden guns and repairs". Obviously they were mostly gunmakers to the trade, making "economy" guns for sale by wholesalers and country gunshops. In 1926 they pleaded bankruptcy.