The only really remarkable feature of the G3SG/1 is the trigger pack. It had a "set" feature, where a button behind the trigger could be used to set up for the next shot, at which point the trigger break was at something like 1 pound of pressure, and supposedly crisper than the PSG/1 pack. However, the pack could also run full auto, making the G3SG/1 one of the most versatile rifles made by HK.
Other than the trigger pack, the SG/1 variant had nothing remarkable about it, save that it was built using hand picked pieces from the G3 assembly line that came out with the best tolerances. Someone else mentioned the MSG90 stock -- that was not fitted to the SG/1. The SG/1 had a modified G3A3 stock with an adjustable cheekpiece.