Glad you too noticed the look of this penny. (You were also right about that Belgian 50-franc piece.) I pulled this '17 penny out of my duplicate box, trying to save a little effort photographing. It's been sitting on my desk since then and I've found myself admiring it over and over. A better one is supposed to be in my collection, but I think I'll check to see whether it has the eye appeal to go with its technical grade.
I looked some at the
OED and recast the entry as follows:
x: A 1917 “cole,” then British soldier-slang for a penny. “Cole” derives from “coal,” says the
Oxford English Dictionary, and appears at least as early as 1673 as old British slang for money in general (a 1771 citation explicitly describes “cole” as pounds, shillings, pennies and farthings.). But apparently “cole” somehow became fastened on the penny, perhaps—and this is only me talking—because of “penny-stone,” a usage the
OED finds as early as 1803 describing a distinctive type of ironstone found in a Shropshire coalfield. At any rate, I’ve read that “cole” as slang for "money" had gone into decline during the 19th century and was extinct by the turn of the 20th, but I’ve also seen that contradicted. Former American soldier Arthur Guy Empey wrote
Over the Top, a widely-read account of his WWI service in the British army. Included in his book—published in 1917, the same year this penny was coined—is a section called “TOMMY’S DICTIONARY OF THE TRENCHES.” In it, “‘
Cole.’ Tommy’s nickname for a penny. It buys one glass of French beer.” (xx)
A few notes (guesses, mostly) about coal > cole = money:
1) "Coal" is an old word in English, of course, and important. Coal is fuel, a lubricant of life, an agent of comfort, a tool of industry. Money is all these.
2) Coal is in fact found money--Pop used to tell us about picking fights with the firemen of passing trains back in the '30s, hoping they'd throw lumps of coal back at them; they'd take it home, or sell it. (And of course the firemen knew the favor they were doing for the kids--and I expect this same mock-hostile tableau was visible in about every Depression-era country.) Anyway, coal
is fuel
is money, and maybe that's the root of the usage.
3) Finally, "cole" as early slang for money in general definitely goes beyond coins (I had no idea the contemporary British paper denominations were so high!)--thing is, the word encompasses more than just money's physical representations--in some of its constructions "cole" also speaks to money in the abstract, as a unit of account.
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Don't know how much of this is right, but it was fun to think about. (And we know a penny bought a British soldier a glass of French beer c.1916-17!) Thanks....

v.