Bistro saucers served a duel purpose in the pre-ban absinthe era. Besides serving as a coaster, they also functioned to indicate the price of absinthe being served. The price was written on the inside rim or bottom of the saucer and was use to advise the customer of the price of his absinthe. The money was generally left inside the coaster itself.
What a lovely housewarming or hostess gift for a total francophile ....like moi!