I recently dragged a set of 3.5 '63-'65 tanks out of storage that I obtained in '75, and observed a drooling tar-like sealer applied to the seams inside.
After nearly a month of acetone soak, many days of of it tumbling with aquarium gravel and acetone, I find that although the sealer softened, it still adheres and does not dissolve.
Attached is a photo of some of the material after it is scraped loose.
(As always, external photohosting sucks; clicking on the thumbnail will enlarge it, but take you to imageshack)
Before the acetone soak, it resembled the adhesive used to secure the felts in Panhead valve covers. Anyone know what the felt adhesive compound might have been?