Interesting. Finding this.
Refers to a primer which contains potassium chlorate.
Refers to a primer composition containing a mercury compound. Non-mercurial primers are desired because traces of mercury in a fired cartridge case make it brittle and less useful for reloading.
Here's a site that gives a lot of information (sometimes the grammar isn't good) - in the primers section.
http://www.aeroballisticsonline.com/bal ... lants.html
In general, mercury works better, but damages cartriges. Potassium Chlorate dosen't hurt cartriges, but can damage the barrel.
Intermediate primers that were non-corrosive, even with the addition of mercury (with something to keep it from damaging the cartridge) tended to overstabilize after a year or two, and go inert.