Dave, when I bought my 65 I had them put in super soundproofing as part of the prebuy / annual process. Good thing because above the pilot seats on the roof, underneath the old factory soundproofing was corrosion. I understand this is often the case with that old soundproofing material. They were able to scrape, sand and repair but I would have had a "moonroof" in a few years ! Other than that it was corrosion free. So... check under the factory soundproofing for sure. Never would have found that in a normal prebuy or even an annual.
I thought there was a list of prebuy items on this website but I cannot locate. You should get the 337 book from this place
http://www.skysmith.com/
Best of luck;
Brian