While searching for one of the accessory codes via Google, I stumbled onto this site.
I was looking for the 5Y that is on my cowl tag. In the above site, they call out 5Y as deluxe seat belts with front retractors. this now makes sense for me as deluxe belts with retractors were found in one of the boxes of parts that came with my '66.
Now if I can only find out what the A after the 842 means... I know what the 842 is, just not the A.