Two-inch rail is the size used where the load is real, and it is also the size most likely to land in a tight spot: an inside corner, a column face, or a pair of flanges sharing a post. The 531/2 is the 2-inch wall flange with the base trimmed for those conditions. The collar is the same as the standard 530/2 and the set screw locks the tube the same way, so the fit and the strength do not change. Only the footprint does. Choose polished brass to match a period guard rail or brass cap rail, or 304 stainless in polished or satin for interior runs that see daily cleaning. Mix it with the standard round-base 530/2 across the same run; the collar heights match, so the rail line stays true. Ships from US inventory.