I have a CZ MODEL 612 target that came with 4 stock shims. They are marked "D1", " D2", "U1", and the 4th one has a " R" on one side and a "L" on the other. I think the "U" and "D" stand for up, down, left and right. Can anyone tell me what these mean in terms of adjusting the POI of the gun? I have called and emailed CZ customer support with no luck. Thanks in advance.