SPD = Scheduled production date. The flat pad is on the bottom of the motor on the back passenger side oil pan surface. The flat pad on the passenger side of the block DOES NOT have VIN stamped on it for the '68 models.The Chasis number is the VIN number and it is stamped on the radiator support and the trunk lip, under the seal... That is what I have learned on here in the past few months. I think that is all accurate info., if not somebody will correct it.