The Plain Mold (PMSC) chips like Harrah's and Harvey's may have been made by Reliable Engraving Co. of San Leandro, CA.

The reason I say "may" is that Reliable made such chips, but there may have been a name change when Mel Jung bought the company. If so, those chips may have been made prior to Mel's ownership and another name. Just not sure.

There are many Reliable manufacturer chips on MOGH, which I'm sure you are familiar with.
Here's one of them, as shown on MOGH.

http://chipguide.themogh.org/cg_showpic2.php?cg=173910

I think it was Howdy Herz that coined the term PMSC, which stands for Plastic Metal Slug Core.
See it in Howdy's book "A Collector's Guide to Nevada Gaming Checks and Chips", page 27.
He notes that it was a technology developed in the 1970's.

Hope that helps a little.