Montenegrin is right. The tight choke in drillings was common to nearly all German shotguns up to the 1960s. The main purpose of shot barrels in Germany then was shooting hare, sort of a large rabbit living in open fields. This is also the reason why most German shotguns and Drillings are stocked rifle-like, designed for shooting mostly downhill. Shotguns were rated by tight patterns giving range, the tighter the pattern, the better. And, remember, this was still the time of paper cases and felt wads, modern loads with plastic shot cups need much less constriction for tight patterns. As formal shooting at driven feathered game was almost unknown to most German hunters, the advantages of an open-bored "game gun" or "upland gun" was not seen then.
BTW, original Brenneke slugs were designed to be shot through these tight choke bores. Brenneke's older advertising even claimed best shooting through full-choke bores.
