Honda GL1800 Service Manual page 6

GENERAL INFORMATION
The frame serial number is stamped on the right side of the steering
head.
The engine serial number is stamped on the right side of the
crankcase.
ENGINE SERIAL NUMBER
The Vehicle Identification Number (VIN) is located on the left side of
the frame near the steering head.
EHICLE IDENTIFICATION NUMBER
The throttle body identification number is stamped on the rear of the
throttle body.
THROTTLE BODY IDENTIFICATION NUMBER
1-3