Out of 4 old pumps I have, 3 have a screw head bolt there, one is a 10mm hex head. Send me a message with your address and I'll just drop it in the mail. These are obviously some kind of guide for the plunger as you thought. They also have a left hand thread.
As far as testing, hook an oil line supply to the center port. The supply should be above the pump, and bleed the air out of the line. Spin the shaft CCW (looking at the shaft) by hand, see if oil comes out the two other ports. You should hold the lever full open to see quickly if it is pumping. It pumps such a small amount per revolution, it will just ooze out. You could probably use a drill at about 1000 rpm to get quicker results once you have oil in the pump.
