Bob and List,
The Vickers type p/s pump is mounted in a variety of M-B:s (and other cars/light trucks). Anyway, many of these installations allow the ATF reservoir to be placed directly onto the pump housing. The hole you ask about is then used in order to mount the reservoir.
In the case of our W113's, the reservoir had to be mounted higher up, and a hose leads the fluid down.
So, rest assured, all is well with your p/s pump...
/Hans in Stockholm