I don't know that code. However that is a Briggs engine on that tractor. The engine I.D. Tag is somewhere on the engine blower housing. There will be three numbers on that tag.
You'll see it's a Model 251707 Type 0391 After those numbers will be a Code Number. The first 6 digits od that number give the manufacture date of the engine. First two digits are the Year, next two digits are the month, next two digits are the day. The remaining digits are the plant code where it was made.
For example a Code number of 99031523 was manufactured in 1999 - 03(March) - 15th
Rick, April 2006