I think Tecumseh did explain in their workshop manual how to date a lot of the later engines they made before they went belly-up. I don't know if Dave's white one is late enough to be included. Dave, if you post the serial number which has a few letters at the beginning, and the other letters and numbers from the same block of data, we can see if it features in the manual, and whether it contains a date code.
Tecumseh engine H30-35311S Ser 0151B.
