After the horror of being forced to imitate a Huret Allvit with the Schwinn GT100, Schwinn allowed Shimano a bit more freedom with the design of the Schwinn GT120. The GT120 is basically a Shimano Lark STO with Schwinn touches:
Shimano did take the opportunity to build-in their latest adjustment screw layout - with plastic shake proofing instead of steel springs.
This example has two sets of date code letters stamped on it (AC and YL) - holding out the promise that it is the Rosetta Stone of the two Shimano year letter systems - but it still does not quite make sense. I think A is 1976 in the ‘new’ system, Y is 1979 in the ‘old’ system. I have gone with 1976.
One day it will all become clear.