The 6-digit serial number of all P-line gearboxes is engraved on the top right-hand side of the housing at the rear edge. In some cases, the number may be difficult to read when the gearbox is installed, as part of the bridge to the chain stay covers this area. The serial number for C-Line gearboxes is located below the gearbox mount on the rear of the gearbox. The 6-digit number begins with "C". This photo shows you where the number can be found. (Attach photo)