matching numbers

Have 68 cutlass S convertible. I have found numbers on the oil filler as described in the service manual. I also have found numbers on the block casting, behind the water pump. The vin and ID plate on the firewall are visible. What do I compare to verify matching numbers?

Welcome to AF.

All of the numbers need compared.

Try here...
http://www.ucalgary.ca/~csimpson/TheOldsZone.html

The vin # on the block is located on the driver side just below the #1 cylinder.
The first digit should be a 3 for olds and then an 8 for 1968 and then another letter for which assembly plant built your car. The last of them should match the last 5 or 6 of your vin in the windshield.