It's the name of the set, SA 1976

As someone else mentioned, the set is called 1976, to expand on this, it was called like that because it was inspired by a 1976 olympics poster or something like that if I recall correctly, it had the same font as the 1976 on the deskmat and the same colors as the set. IMHO one of the most beautiful SA sets, too bad is not in stock anywhere...