I think the number 5 relates to the size of the shoe. You have a hind shoe with a toe clip, a toe grab, and heel caulks, pronounced "corks." Horse shoes sizes start at 00, or "double ought," then 0, which are small, and as I recollect, the size 1 and 2 are the most used sizes on light saddle horses. Size 5 would be for a large animal. The corks and grab are for traction which tells you the horse was pulling a heavy load, and the the toe clip, which was heated red hot and burned into the horses hoof wall, was to keep the strain of that extra traction off the nails. Amish still use horses shoes exactly like that one today, same with people that horse log. Trying to date a horse shoe by shape is impossible. It doesn't look to me like your shoe was on the horse for very long. More likely it's not very old. I used to make shoes like yours, only smaller and without the toe clip, for people to use as a trivet.