That mold name, and others first appeared in the 1985 book "Harvey's Guide to Collecting Gaming Checks and Chips" -- by Howard Herz. There are about 60 different molds named & pictured in it.

You will see a cross reference to the Herz mold codes in the front of TCR. See page XIX in TCR-20. Additional molds have been added over the years, to include the extensive list of mold codes in ChipGuide.

(Borland's "Blue Book" may have had codes as well. But my copy is not handy right now.)

With 35 years of calling them "squares" (even though I agree they're more like rectangle/trapezoids), calling them DiaSq works for me.