Winchester 333 Questions

Underneath the 2 side caps on the body of the rifle are 2 rubber plugs in the stock. What is the purpose of these? To retain the caps perhaps? I'm referring to the Winchester 333 but I think it pertains to any Giss system airgun similarly made. Also, what is the purpose of the large screw underneath in the stock forward of the trigger? It threads into the wood, not the body of the gun itself.
 
The screw forward of the trigger near the forend tip is a place for the two piece cocking arm to butt up against, so it won't be able to wear away at the wood as it tries to invert itself during cocking. If you remove it and look at its tip, you'll notice it is burnished by the underbelly of the rear segment of the cocking arm. The two piece cocking arm is a means of reducing how much wood is relieved for the cocking arm, for cosmetic reasons. 
 
By the way it is important to screw it in the right amount... NOT to a "hard stop". No need for ever removing unless absolutely necessary. The idea is to dial it in just far enough to keep the mid pivot of the two cocking arm segments above the arc of movement which prevents them from inverting and thusly bending downwards towards your stock during cocking. Conversely screwing it in too far causes the rear segment of the two to bind against the receiver tube.

Hope that made sense.
 
Wadcutter nailed it !!!
The pinion caps are tightenned up pretty well during assembly, but because they double as the axle for the pinion... AND are affected by the spring TWANG which is smack dab up against the pinion whereby the only thing holding the pinion on its own axle is the fact that it is leaning on the spring... the rubber takes over should the caps ever vibrate loose.