picture of inside of a Pertronix dist.

The tan material (glass-filled alkyd) was an upgrade from the black material (phenolic resin, "bakelite"); it is less water-permeable and has higher dielectric strength. That is why Chrysler phased in the tan caps across ~all applications in the '64-'65 timeframe. No TSBs that I've ever seen, but enough better to comport with "The policy of Chrysler Corporation is one of continual product improvement; specifications are subject to change without notice".

Whether any given car got a black cap or a tan one was down to what was in stock/came to hand when a particular distributor was being built. You can follow the part number supersessions for any given type of cap through the years, and it goes black-black-black-tan-tan-tan. The next material change was in the mid-late '80s, when glass-filled thermoplastic caps began to replace the alkyd ones, for the same reasons (plus thermoplastic was cheaper, and easier to ventilate in a splash-resistant manner).