As I read it, the top rail is either CD 137603, or GD 137603 (I believe this is the correct one as it matched the VIN), or 6D 137 S03 or some combination thereof and the side of the left rear rail says:
12 30 65
I believe this is the correct one as it matched the VIN
You are correct. What you are looking at is one of the "hidden" VIN numbers that were stamped on the car as the frame was about to be joined to the body that would "hide" it. This was a hand process (someone had to get a bunch of dies and put them in a die holder so that the combination of letters and numbers were all aligned. Then smack the handle of the die holder with a single hard smack from a FORD hammer to leave that imprint in the steel frame.
If you had an engine that was factory rated at over 300 horsepower (usually a BBC though some Corvette small blocks crossed that boundary) it would have the last six VIN numbers stamped on the identification pad on the front of the motor. This would have happened even with a base motor from 1968 and up, because after 1967 all cars had to have parts stamped with a VIN to reduce auto theft.
The frame was stamped with a hidden VIN (as was the body) after 1963 under a treaty with Canada.