Thanks William and Bill. Great Information! If I were to use my portable blasting pot for the soda blasting, are there any special nozzles or tips designed for soda or fine material? Does the air pressure used make a difference? Are the grease penscil marks etched enough that you don't have to worry about removing them, even with using soda, or is this a babied process?
As Austin stated, there has been a lot of recreations to the firewalls on restored cars. And now, especially after seeing the options found by these markings, color codes, etc. It seems to me the best choice is to document the markings with digital photos for the restored album of what you had and where, and be done with it. Paint the firewall back like it was, install your PBT back in the same spot, and don't add something the car didn't roll off the line with! But, I see the beauty of this as documentation that you didn't have if you didn't find you build sheet under the back seat, which I didn't!
I think it would be cool to start a registry of these findings to see just how in depth it was. (official CRG project addition request) How many options have been found on one firewall? Has anyone found NC8 or JL8, etc. It appears there has been interior and exterior options found, what determined why it needed to be there and other options were not? What if the body was to be a heavily optioned car, how would that work out?
Why did the assembly line need to do this? Was the build sheet not with the body yet?