That appears to be a 1970's Japanese manufactured Schwinn serial number. The letter corresponds to the month of manufacture (i.e. A=January, B=Feburary, etc), L being equivalent to December. The year of manufacture was indicated by either the first or second number following the letter, so 1972 or 1978. The badge plate on the head tube (should be oval shaped and say something like "Approved Japan" at the bottom should have a 4 digit number stamped on it. The first 3 digits of this number are the number of days into the year that the bike was manufactured (your's should be be around the 330-365 range) and the fourth digit will again indicate the year, so if the last digit on the badge is a 2, then 1972, if it's an 8, then 1978.
If anyone can double check me on this, it would be appreciated.
-Josh
2003
The Schwinn bike serial number H19577 was made in 1951.
August 1974
1984
November of 1981.
June, 1967
85
FEBRUARY 1974
I just searched for the serial number of my Schwinn Collegiate. From the same source it appears that mr850439 was manufactured in December 1980. Here is a reference: http://www.bunchobikes.com/serialnumber5.htm
If the serial # is on the headtube, that bike was made in Chicago in October 1975.
1977 would be my guess
Bk=Black 52=model# 1954=year of manufacture