engine identification

Hey guys, does anyone know how to identify the year of a tw engine by the model number? I just acquired an engine with no information about it. The PO had no interest in TW's and had bought it off of a guy who scrapped a TW because all he wanted were the wheels. shame.

Anyways, the only number I can find other than the 196cc is 15x-020576 and this is on the upper right hand side of the crankcase above the oil fill hole.

Can anyone decipher the number?

Thanks