Really old Compressor, need help



I have the compressor picture below. It is functional and providing ~20-25 PSI at its current settings.
My question is what is supposed to go int he hole that is in top top of the head, currently filled with a screw.
The hole is "spring Loaded and air seeps at around the screw. Was this where the gauge/regulator is supped to go?

Edit: sorry for the upside down. can't figute out how to fix it. Xlick and it'll tkat you to photobucket with a proper aspect.

Cant help with your compressor but let me welcome you to the forum, you should head over to the intro section and tell everyone a bit about yourself were your from and the like. Just thinking if your compressor works as it is then use some plumbers tape or something to seal the leak and spray on
That screw replaces a plastic seal screw like this, it should be sealed completely so you can get your max 45 psi

I alwAys assumed this was some kind of a pop off protection and this pictures seems to confirm it. Any idea where to get the plastic seal?

Since the compressor is so old it could be hard to find, if the screw that is in it now fits you can wrap ptfe plumbing tape around it for a better seal or you could check out different size sealing end plastic screws used for sealing unused openings on outdoor electrical installations.