For context, this is a (rather old) Cullmann 2502. The head features a slot (30 x 5 mm) with two set screws on the front facing side (see other pictures). I thought it could be a mounting point for accessories but could not find any info online...
Front face with set screws:
Full tripod:
I'm giving you a wild guess, but I suspect it's to mount an arm to hold a flash, alternatively it's for a handle so the head can be rotated smoothly with a video camera mounted.