If I understand the screw location, there is a screw on the pump that holds/adjusts the flat rubber cam against which the impeller compresses, forcing the water out of the pump. It is not, per se, a lubrication point as you would be lubricating the back of the plate, never reaching the wear surface. There are many diagrams of the pump on-line. Look for Jabsco Model 5320-0011. It will be item #9 (the screw) pushing against item #5, the cam.
Should you need a copy of the parts list, repair instructions and pertinent drawings, e-mail me and I will send you the PDF. Note that this is the pump on my 1979 Westerbeke 50. There are, potentially, others that were used on earlier or later models. This pump is, however, used on many motors and the construction of others is quite the same. If you want to lubricate the impeller easily, put some white lithium grease into the intake side of the pump and start the motor (having reattached the inflow hose).