1969 Camaro - possible Motion

[PhaseIIIZ28]

I did get to crawl into the back of my car tonight and look for the crayon markings on the rear seat support. I expected to see an X6, verifying the X66 trim tag and I took a few pictures. I didn't realize until I got into the house that there was actually X66 375 marked below the glue. Cool - I have never seen the engine designation written, only an X6, X3, etc, or even a DG3 as is the case for my 69 Z with Dark Green interior. Has anyone else seen markings with an engine option noted? I'll try to get some better photos tomorrow.