In (Sir Charles W. Shults
III):
It's a solvent perhaps- probably the "plasticizer" from something
it was in contact with. Ever use a rubber grommet in a plastic project
box? After a while, the plastic box dissolves and the grommet gets
stiff. Same concept. Also, for the newbies- never store those rubber
grommets in your clear polystyrene plastic parts drawers- they too will
melt into goo after a few months. Once that starts, there is no way back.
Cheers!
Chip Shults
I'm thinking it is something like this. The pouch seems in very nice
condition, it is not stiff or old but if anything quite pliable for
reinforced vinyl of late 80's vintage. The affected area is quite supple,
and the textured surface does not appear marred in any way.
If the sticky stuff is going to be absorbed by the vinyl, it could hury up
already!