The first two digits of a Briggs Model Number are the engine's displacement in cubic inches, so for a 15.5 hp version they will be 28 or 31. However I believe there are problems with some of the other digits you've given us as well, since the Briggs website assures me no such engine exists. There is a 311587, but it seems to be a special purpose engine for use on pressure pumps.
The Type you've supplied doesn't feature in their records in conjunction with that Model number. Note there should be a dash after the first 4 digits of the Type, followed by the last two digits.
If the Code you supplied is largely correct the engine was made on 15 August, 2006.
