The Classics are not snap-on tools ( :D ), and the things that determine quality are the plastic used and the accuracy of the mold. I am guessing that some plastics mold better than others, and for sure some hold up better than others. I have had old nocks break, resulting in an "oh spit!" at the firing line as the bow dry-fires and the arrow goes astray. And some of the newer nocks just plain sounded brittle, right out of the bag. It was the type of plastic they used, the nocks tinkling when shaken rather than thudding dully against each other.
The "bad" Merc nocks will have uneven edges around the mouth and prominent mold lines lengthwise. They are useable, but that is about it. I love Merc nocks, and have a lot of them, but the non-snap-on Bohnings are very nice indeed. Sleek.
Killdeer