I do have some part pumps here.
I suspect you have an R18 series pump.
If it has an "Oel" cap, it has a dip stick. Or it has
been lost and plugged. [Rear of the pump, T handle]
A pump # will tell. The Bosch # will be the last series of #'s. R18, R 18y, R18z , etc.
The force feed engine oil pumps without dip stick did not come out until the R 20 pump. [ 280sl]
Though this may not be your problem , I would be concerned to find out if the pump does in fact have any oil in the res.
If you find the pump #, I will check and see what I have in used parts