P-line gearboxes always have a 6-digit serial number. This is engraved on the upper edge of the left-hand housing at the rear edge. In some cases, the number may be difficult to read when the gearbox is installed, as a part of the frame bridge to the chain stay may cover this area. 

The gearbox type is also engraved on the left side of the upper housing, closer to the front.

The serial number on C-line gearboxes is located below the gearbox mount at the rear of the gearbox. This is either 6 digits long, starting with a C, or 15 digits long, starting with the product number. The photo below shows you where to find the number. On some bicycle models, this area may also be partially or completely covered by the frame.

On C-Line gearboxes, the gearbox type is usually found on the cover of the gear control on the left-hand side.