A bit more about Raymart/Radiomart here:
https://www.radiomuseum.org/dsp_hers...mpany_id=18816
And here:
https://www.qrz.com/db/GI5NI
I remember the Raymark Absorption Meter 'Band Checker' with fondness.
Poke it towards your homebrew transmitter, which was spraying RF all around your shack, all over your walls, your curtains and your neighbour's TV, (and hopefully some up your aerial!), turn the knob till the MES bulb lights up from the RF, and it will sort of tell you what band you are on, but not what frequency or whether you might be outside the edges of the band. It would also tell you what harmonics you were radiating on.
The marvels of science!