I was searching through eBay.fr for a bar spoon. I decided to try the French word for spoon cuillere. This turned up thousands of listings. Most were boring. Some were interesting (the cuillere glacons ice spoons that appear to predate Kentucky Fried Chicken's spork). And one was astonishing: the cuillere medicament. This 17th or 18th Century French apothecary spoon looked so much like a modern bar spoon I thought it was mis-listed at first.
Then in a reprint of the 1898 Farrow and Jackson catalogue (Thanks Jeff M.!), I found a page offering a choice of a regular bar spoon, or a French mazagran spoon. Mazagran normally refers to coffee-related sundries (cups, spoons) in French, and the name comes from a coffee town in Algeria. But this former apothecary spoon, in service as a coffee spoon according to its name, was being sold to British bartenders as a bar spoon.
The origins of the bar spoon? Perhaps.