I can only speak from a singular experience when I had the subframe out from under my 69 RS back in 2004. I looked all over the subframe and did not find any markings, numbers, etc. Quite different from GM A bodies that I've been involved with ('66 Chevelle and '67 GTO) where there were stampings and numbers all over the frame, including manufacturer, build date, and partial VIN - but alas, nothing on the '69.