This vintage 1961 assembly manual is for the Heathkit Harmonic Distortion Meter Model IM-12. It provides step-by-step instructions for assembling and using this unit, which was manufactured in the United States. The manual is a valuable resource for anyone interested in test equipment and measurement. The manual is specifically for the Heathkit IM-12 and includes detailed diagrams and explanations of each step in the assembly process. It is a great addition to any collection of vintage manuals and would also be useful for those restoring or repairing similar test equipment.