Hello.
The subject is old, but it may interest people.
I have the same model, but from 1959, also with the GPD governor.
I could read on documents, that the filling is done by the side flange with the 8 screws, which must be dismantled. Normally, under the injection pump there is a bolt with a pipe going down, on which there is a loop, which must serve as a drain, but also as a level, since the pipe must have a certain length inside . As soon as you put oil in the pump through the hatch and what comes out through the end of the pipe, the level must be good.
In my photos, you can see that my drain hose has been modified, but I will try to redo it to the same as the original.
Additional information, if diesel comes out of this pump pipe, there is a good chance that the primary pump diaphragm has cracked. I currently have this problem on my machine and I am waiting for the delivery of a ULPK0006 pump, either to change it completely, or simply to replace the diaphragm of my supply pump.
Have a nice day.