Quoted from Wickerman2:
I do the same...my "green drop target" plastic is broken in that spot and I think the plastic for the longer target bank is cracked. Be nice to have a set like pinbits does for the newer machines.
I have used the clear plastic washers to protect, but also they raise the light shield higher which may also protect it from the ball. Regular small washers can also be used on Gottlieb metal posts to raise the light shields.