Gustav Genschow & Co. AG, tm “Geco”, Berlin, founded 1887, was a German ammo maker, wholesaler and dealer with factories and outlets in several German cities. Though they made many .22 smallbore rifles themselves, they had their “better” guns made for them by the Suhl, Zella-Mehlis and Liege guntrade. Post-WW2 they came under the control of RWS who still use the Geco trademark for some economy ammo lines.
This clamshell double rifle was made for Geco by Ernst Kerner & Co, Lutterothstr.7, Suhl. Founded 1892, the company was finally closed down by the Soviets in 1947. Ernst Kerner, 1867 – 1939, invented, among other improvements, Kerner’s improved Anson type boxlock with the top-hung sears. This design later became the Suhl standard for boxlock guns and is used to this day. On guns he made for the trade he used that ornate K mark instead of his usual capercaillie / EKO trademark.
As you have noted, this double rifle was proofed in Suhl December 1927 for use of a Nitro = smokeless load with a steel jacket 26 gramm = 401 gr bullet, most likely for use of the contemporary DWM load: a 26 g bullet in front of 5.2 g = 80.2 gr Walsroder 1919 powder for a mv of 710 m/s = 2330 fps at 3150 kg/cm² pressure, 28” barrel. The 73 (mm) stands for the case length, but the 10.4 mm neither for the bullet nor the groove diameter, it stood for the narrower bore (before rifling) or land diameter of the barrel. Other marks are unidentified individual ones of the several craftsmen involved in the making, like barrelmakers, actioners and so on.
As you may see from the screw at the rear top of the action body and no screw at the rear bottom, the rifle is built with Kerner’s top hung sears. These allowed to move the sear detents as far away as possible from the axis of the hammer movement. So the mainspring applied much lesser pressure on the detent, allowing for lighter and safer trigger pulls than the original Anson Deeley design with the bottom sears, engaging the hammes very close to their axis.

The clamshell action body design served the same purposes as bolsters and side clips on other break-open actions, but more efficient: It strengthens the right angle from breech face to action bar and takes up the sideways stresses when firing side-by-side barrels. This strengthening of the action body without adding a lot of weight made it popular in Germany for double rifles using high pressure loads. Fitting the barrels to such a clamshell action was much more complicated than fitting them to a conventional water table. This made clamshell actioned guns more expensive than conventional break-opens.