> So my question is, is this RTV?  If not, what is it?

It is most likely PVC-E glue or Phenoseal.  One was supposed to
be a glue for plastics to wood and the other was a sealant used
on basements up north.  Both are a pain for a good restorationist.

Unfortunately, at this time only nylon cloth is available for
pneumatic recovering, so PVC-E glue will be found in the future.
Regular [rubberized] cotton cloth is not available from any supply
houses.

Don Teach
Shreveport, Louisiana