I have observed on most original car that have been certified through the Vintage Camaro (survivor class at Carlisle) to have some degree of exterior color overspray on the underside floors pans. This overspray has been found on both the sealant and plugs. I have never seen primer on the plug or sealant just body color. Another item to note is that the adhesion property of the plated plugs is very poor and in many cases some of the paint has come off. Also the degree of body overspray on the floor boards themselve varies quite a bit, therefore varying the amount on the plugs/sealant. Remember this is not a black and white issue as I stated some cars will have alot of overspray and some may have very little if any left after 38 + years.
I will review my photo files and look for very clear examples, one that comes to mind is Don S.'s 69 HO Yenko. That car had an amazing underside, that was very well preserved and showed the overspray very well.