You can find out more info by visiting the Vintage Trek website and click on Component dates. Scroll down to Suntour and start your investigation. I have a Suntour front derailleur and it is stamped YC, with this info, as long as the component is original, the bike could be a 1982 or 1983. Here is some more info on Suntour:
In 1982, the year before mountain bikes became significant, SunTour equipped about 60% of the medium-priced bicycles, compared to Shimano's 30%. SunTour had 13 different lines of derailleurs. From the top they went; Superbe Pro, Superbe, Cyclone Mk II, Cyclone, BL, ARx, Vx, AR, Seven, Volante, Honor, GT, and Mighty Click. Most lines had a racing and a touring model. Vx rear derailleurs came in four different capacities.
According to Sutherland's, the AR "medium cage" can handle up to 28T freewheel, has a maximum total capacity (difference between smallest and largest freewheel cogs plus the difference between smallest and largest chain rings) of 30T, and can accommodate a 16T maximum chainring spread.