I’m fairly familiar with old electronics stuff. I’ve been around it most of my life, and I’ve been researching and writing tech history pieces for 10 years.
This one is a complete puzzle. It’s an attractive deco ad in a 1930 issue of the British Popular Wireless. The text doesn’t say what the Blue Spot is. Clearly it comes in two versions, the 66P and the 66K, with a slightly higher price for the 66P. Both versions seem to include a permanent magnet, and both things seem to have control knobs. It sort of resembles a coherer, but 1930 is beyond the pull date for coherers. Everything was tubes by 1930.
The lower thing also resembles the active ingredient of the GenRad string oscillograph, but I don’t know why you’d want to buy that on its own.
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Later, found a detailed ad in another issue of Popular Wireless.
It’s a speaker driver.
Polistra's Mill 