Just looked up option code J55 and it states "undercoating w/hd pad" so this makes me believe it is being mistaken for rust prevention undercoating when it really sounds (yeah a pun) like it is noise reduction when it includes an under hood sound deadening pad. No way would they include an under hood pad with rust prevention. Also if it were rust prevention undercoating then they surely didn't do a good job by spraying the most likely places where rust starts in the salt/snow/rust belt IMO. J55 should have been named "extra sound deadening undercoat with under hood sound deadening pad" Another thing for example that this "undercoating" would be done differently at each assembly plant is if you take a look at the in trunk quarter panel SOUND DEADENING and how some plants like the LA Assembly Plant put way more on the inside quarters of a B-body than a Lynch Road Assembly B-body. Using one example from one or two restorers that are from the same assembly plant will not tell the story of how to do your car. Best example of differences are the in trunk sound deadening and the jack instructions on a '69 LA plant B-body versus a '69 Lynch Road B-body.