Number on B&S is on the cowl, should be just near the carbie/air cleaner on your mower. Look at the last batch of digits(code). First 2 digits of code is the year, second 2 digits month and third 2 digits the day.
Example if code is 88081203, engine was built on the 12th of August 1988.
This explains all the numbers.
http://www4.briggsandstratton.com/miscpdfs/RNT/Numerical%20Designation%20System.pdf
