Good idea Minuteman, just posted the same guess at Benno's a few minutes ago, but didn't notice the odd box until I read your post here. Did a quick search of "War of Spanish Succession horse" in the web but didn't find any matching pictures. Some Dragoon regiments had a company of grenadiers, who did carry a cartridge box this way, but these almost always had distinctive headgear (viz. no tricornes). Seems an odd placement for a cavalryman. Perhaps some cut way stuff? Otherwise they are quite nice. Let's wait what Strelets has to say about them.