The 4F is not a serial number but something else in it,s assembly I think from the factory. The SNs are both the same as you posted engine and tractor back when it was made. If the engine was different then the only way it could change is if the engine was replaced from another tractor or machine that had the D315 in it. I don,t see where he gets the SN unless his parts he was looking up showed covering those higher SNs.