What the heck is (was) this?

[terrywerm]

I worked for a few months in a rubber products manufacturing plant many years ago, and I can say for certain that what you have there is exactly that: a heating platen for a molding press. I've got one here that I cut up into pieces to use as small suitcase weights for a compact tractor.

Each press has two platens, bottom and top. Steam is circulated through them as a heat source. The two halves of the mold are then attached to the platens. Once the press is hot, you load your raw rubber into the mold cavities, then close the mold and wait for the appropriate period of time before opening and extracting the finished parts.