That's a more complex (or more subjective) question than it first appears.
Crest & Seals do not have a mold (like Hat & Canes, etc.) so they would be considered a "plain mold".
However, personally, I don't consider all plain mold chips to be Crest & Seals.

I think of C&S chips as those made through the 1940's-50's by US Playing Card company (USPC), plus some of the older Burt plain mold chips. You will see more recent chips called C&S though. One distinguishing characteristic is that the inlay is sealed under a clear top coat that goes edge-to-edge. That is, if you run your finger across the chip, you won't feed an inner edge where the inlay starts.

But others consider all plain molds as C&S. Can be confusing.

Rich Hannover's excellent site shows C&S's through the years. Take a look here:
http://oldpokerchips.com/CSdates.htm