Wick-
I have often thought about how many households had a gun, and wondered why there aren't so many more of the originals. I suspect that it has to do with the prevailing attitudes, and that these were seen as tools, to be used and then turned into something else when worn out or used up. In that vein, a loading block would end up in the trash bin or woodstove when worn out or broken and, being made of wood, wouldn't survive being outside or otherwise unpreserved for more than a few years.
so, maybe we're in the scenario of virtaully everyone had one, it was unremarkable, and they don't last that long anyway, like a modern tooth brush- if it weren't for advertising, would we know about them in a few centuries?
just a thought