Questions on Heat Audio instruments

[Malcolm Leonard]

Down through the years,the Benton Harbor Boys offered an audio analyzer and a separate harmonic distortion meter.
My 1965 catalog shows them as the IM-12 and IM-22,but they were known by several other model numbers at various times.
I am trying to learn the history of these two machines and how the various incarnations may have differed from each other in tube line ups,features,etc.
Also..has anyone here built/used these and if so,how did you like(or dislike) them.
I intend to use this infromation to decide on purchasing these or not.
Thanks,
Malcolm Leonard

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