It's a BFO and dates to no later than the early 1970's. If I remember correctly I gave a relative $3 for the one I had. The plug on the coil wire was bad when I got it, but it was an RCA type and after replacing the plug it worked. It was fairly stable, the threshold didn't drift much, but not much depth. A three inch deep quarter would barely give a rise in the threshold and the ground here is virtually mineral free. You're right about it being more of a collectible, but finding a buyer for it could be a problem.
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