I think Jimmy V said his car is a pre X code, and has it behind the seat.
Here's my crayon mark behind the seat; Oct 68 build 10B; VIN 516355. For a long time I too was under the impression that only cars with the X code on the trim tag had the mark behind the seat. Now I believe it was done from the start of production. The mark supposedly identified the build to line workers even before the trim tag was attached to the body.