Before 1965 serial numbers were stamped onto a rear dropout. 
Beginning in 1965  serial numbers were moved to the head tube 
and the following date system was used for all Schwinns except
Paramounts. The letter at the beginning of the serial number 
indicates the month of manufacture, skipping the letter "I."
For example, "A"= January, "B" = February, and "J" = September.
The second letter of the serial number indicates the year of
manufacture (not the model year), beginning with "A" =1965. 
Skip the letter I & O.
Sports Tourer KH017873, for example, was built in October 1972.