First number is the year quarter, last numbers are the year. So your label should have been used on glass produced in the fourth quarter - October-December - of 1986.
During the 70s the year number was represented by a single digit, so '46' would represent the same period of 1976.
I'm not sure how reliable this is as a means of dating an item, though, as the labels are easy to remove and stick on to other pieces, and the factory themselves may not have been too strict with keeping labels current. I've had plenty of items with completely the wrong decade on the labels, never mind the wrong quarter.