Rail that has a "U" designation has a through-hole like the standard rail, but the hole and the counterbore are LARGER. Because of the larger bolt size to secure the rail, a more rigid mounting is possible. This type of rail is only available for a few sizes of AG. See chart below for specifications. Please pay particular note to the "E" and "P" dimensions. On uncut factory rail, the "E" measure is the distance from the end of the rail to the center of the first mounting hole. This can be a critical point if you are replacing an existing rail section. The "P" dimension is the mounting hole center-to-center measure.