filler filler 
A very unknown and interesting little tidbit about 14th century England is that fossils of sea urchins were so abundant and of such a regular size and mass (one pound exactly) that Oxfordshire milk-maids used them as a counterweight for butter scales up into the 18th century. These fossils, which were inexplicably found lying in thousands on open fields far from the sea, were known as “Chedworth Buns,” “Checkbury Buns,” or “Poundstones.” They became so popular that they started being used for weights in all other materials such as beads, wool, or flour. But since different numbers of stones were used for different sale items, trade with other countries was very hard, so a royal statute in 1389 decreed that the official poundstone, or “stone” for short, would be 1 stone for butter, 26 stones for a sack of wool, and 5 stones for glass. That is how today’s English measurement for people and animals, the stone, got its origins.
filler
filler
(via Science Photo Library, The Map That Changed The World, TYWKIWDBI, Scribal Terror)
allhomosapienswelcome 
You surely went for an obscure one! When I did find “stone” in the regular dictionary or etymological dictionaries, they don’t provide the information on weight, nor nearly as much as what you provide. They do confirm that this is a 14th century term though, but nothing more.
Of course the OED is the best dictionary out there and the entry for stone is pages long; however, if you look under definition 14a you find the definition related to your meaning above: “. . . usually equal to 14 pounds avoirdupois (1/8 of a hundredweight, or half a ‘quarter’), but varying with different commodities from 8 to 24 pounds. The stone of 14 lb. is the common unit used in stating the weight of a man or large animal.” The first recorded literary use is in “Earl Derby’s Exp.,” no date provided, but the second is “Sir Perc. 2024” in 1400.
Oxford English Dictionary 2nd ed., vol. 26 Soot-Styx, s.v. “Stone.”
When I learned about poundstones in Simon Winchester’s wonderful book I HAD to have one. Found one on e-bay from Wiltshire, but my offer didn’t get it. I tried again and got one that comes from the “side of a mountain in Iran”. So now I have a Clypeus Ploti along with a few other fossils.