Tom, it wasn't an open mold back then, it was owned by John Kendall/Chipco.
During the time John Kendall owned the Burt Co assets, prior to starting chipco, he experimented with using it for compression molded clay, then later after forming chipco, had it converted to an injection mold.
Period in question is 1988-1991 (and maybe he carried on a little longer), so that would tie in with these chips.