the denomination ... twelve and a half... has an interesting history in the U.S. 12 1/2 is a "bit" as in half a quarter which is also known as "2 bits". It was an attempt to cut the Mexican (Spanish) dollar (Real) into smaller usable parts. The Real was widely used in the U.S. and was divisible by 8 and not 10. As a token this denomination was also often the price of a drink at a saloon. I am not sure how it fits in with Mexican use.