The letter denotes where the pant was built. A is built in Asia. C is built in Canada. See if the seller has a picture of the tag, it should say made in Canada.
HPG14 are NHL-spec in terms of sizing, which I do not believe the HPG12s are. Also, the waist sizing is different.
HPG12s come in Small (30-32"), Medium (32-34"), Large (34-36"), X-Large (36-38") and XX-Large (38-40").
HPG14 come in Fits 1-5 instead. I suspect the waist size is measured per the NHL rulebook, so a Fit 1 is 32-34.49", Fit 2 is 34.5-36.49", etc. Check out Section 3 - Equipment - 11.4 for the other sizes.