According to the list of changes for the 280 SL, the VIN was moved to the B-pillar starting with 280 SL VIN 10704. This was for US cars.
Also there should be a VIN tag visible through the windshield, on the left hand side. This occurred at VIN ending 07114 - also for US cars as a federal requirement for a "visible VIN" - one that could be seen from the outside of the car.