Did you find the shoe in conjunction with other finds that might help date the shoe. It is almost impossible to date horse shoes. One might believe in evolution, but the horses foot hasn't evolved a lick in hundreds of years, so the same problems that caused a horse to go lame in the days of knights in shinning armor apply to the horse today. Any attempt to date a horse shoe by shape and or size is difficult to say the least. If the picture in the book gives you an educated guess, and you are happy with the guess, then that's fine. But I was shoeing horses in the 60's and 70's, and I've made hand made shoes that look just exactly like your find, and I'm sure there are horseshoers that are making them today for one reason or another. It's like finding a knife and trying to date it by the shape of the blade.