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
It is not possible to know what year a Schwinn bike is with just knowing that is has a serial number of SJ080699. You need to know more about the bike before a person can tell you what year it is.
2003
The Schwinn bike serial number H19577 was made in 1951.
August 1974
1984
November of 1981.
June, 1967
85
FEBRUARY 1974
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
Based on the date code for the letters in this serial number, it was made in February of 1975.