Not to be contentious, but just asking: I have seen, I believe in Muzzle Blasts, a picture of a loading block with the date "1757" boldly engraved on it. The writer presented it as genuine. Spurious?
The fact that there are few or none wouldn't surprise me - one split and it's kindling. In the 18th century the British army had thousands upon thousands of cartridge boxes made, but try to find an original today. There are a handful scattered in museums.
Continuing in my Devil's advocacy, the only old illustrations I've seen of civilian shooters show them either in repose, walking, or shooting - never loading. Illustrations of soldiers from manuals show loading, but they used paper cartridges.
The lack of mention is somewhat damning, though.
But more on the subject - friends of mine with large caliber smoothies have loading blocks of 3-5 balls and use them without a short starter. They round all the edges and make a barrel sized recess around each hole to align the block nicely on the muzzle. Especially convenient in cold weather when the fingers are not as nimble.