The GMC CCKW

Hi,
I have just fitted the engine mounted compressor to my Bolster truck but I am concerned about lubrication. The plate just says to "lubricate compressor when vehicle is lubricated" . There is a screw in grease cap but no other obvious lubrication point, but I assume the piston needs lubrication?
Anybody got any ideas?

I don't know about on this model but most have a plug in the base of the compressor if you post a picture it might be a little easier. What we normally do is feed oil off the motor into the compressor that way its changed out with the engine oil. Let us know how you go.

Chris

This is it! I cannot find any holes or pipes which might accept oil feeds.
What do you think?