The English Officer's rifle, (by Griffin) that I am building has the same plate with the lug for a pin through the comb. This is the second of this style I have done, Like Robert, I just eyeballed the center point, center punched it and drilled it... Holds tight with two screws... I am trying to figure out why I would want to drill a hole for the pin??? On the Griffin gun pictures I have seen, I can't see if a pin was used or not.
Just find the point of the heel, Then screw it into the butt so it pulls both the tang and the plate into the wood. The bottom screw will help you out too..