Sorry for the post. I just found out myself. It's a part of a hinge: LEGO PART 3149bc01 Hinge Plate 2 x 5 with Hole in Stud Holder (https://rebrickable.com/parts/3149bc01/hinge-plate-2-x-5-with-hole-in-stud-holder-complete-assembly/), with the hinging part broken off. Sorry for asking this.
Can anybody identify this part? It's a red 2x4 plate with a small thread hole (about the size a hole in a threaded hook is) in one of the short sides between the studs. The hole is molded in (see bottom), not drilled. I included a top view to show this is an actual Lego part. My Lego collection runs from 1972 to 1988, so it must be between those years or close to it. I tried searching the web for this part, but I cannot find it anywhere. Has anyone seen this part before, and in what set?