Permatex is just a brand/company name... they also make RTV. Are you perhaps referring to their Permabond Non-Hardening Sealer or a similar product?
*shrug* the tube I have is older than I am (yeah, around here we do not only ancestral tools, but ancestral MATERIALS), and is devoid of legible labeling. If I had to describe it as anything, it would most definitely be a non-hardening sealer (or 'black mystery substance that should totally not be eaten')