Horsehide is just that the hide of a horse treated like leather... Actuall shell cordovan is not hide at all it is two oval shaped calluses under the skin of the horse on it's rump. Shell is most often used for high end shoes. It is not actually the hide which is a common misconception but a layer between hide and the flesh of the animal, it's use was much more common prior to the 20th century when horses were more common, for strop makers it has gotten to be prohibitively expensive to purchase in the length required for a strop >20" as the two "shells" found on a horse are often too small. Cordovan is also a treatment used on leather which is often confused with shell cordovan often intentionally to increase price. At least this is what I have found to be the case in my research which was certainly not exhaustive but was fairly thorough. I knew of at least one company still using shell but that was almost a year ago an I do not know if they are still operation. But I think there is only one company in the US making shell cordovan and they clearly mark each piece as such.
I did not know that shells were used for Strops thanks for that info. Every thing he just sad is true. There are only 3 tanneries that I know of that still make shell. One in France, one in china and Horween in Chacogo. Horweens is vastly superior, they have been makeing it for over 100 years and two they only use domestic horse stock. We don't eat horses like France and who knows what china does to get there horse stock??? Horween has nice big shells, thes are 28-36", but they don't come cheep these costume 150 ea