Metal detectors a person used to have and was fond of, get deeper with age as long as you don't actually still have 'em.
With the 8 inch searchcoils, the 400 series were good for about 8 inches max in an air test on coins. And, you can still buy one new (sort of)-- a couple of our industrial metal detectors are repackaged 400 series machines. Not many electronic gadgets are still in production after nearly 40 years but these still are, our industrial customers like them.
The 400's were designed by a British (or possibly Australian?) engineer, Jim Jones, whom I met once (very tall guy). The design was subsequently adapted by Jack Gifford (then of Fisher) to become the discriminating 500 series. Jack left, and later formed his own company, Tesoro. Fisher then hired another design engineer whose name I don't remember (I never met the guy) but he seemed incapable of producing anything and he and Fisher parted ways.
I was hired in '81 and my first product (introduced in 1982) was the 1260-X. .......That project was an odd story. Design of the revolutionary mechanicals was underway before I showed up, they had no idea what sort of electronics they were going to put in it, and I was hired to invent some guts to put in it. Another odd part of that story is that I spent several months trying to put a good static pinpoint circuit in it but they all drifted too much, so I gave up and we decided that the 1260-X didn't need a static pinpoint mode anyhow-- at the time, a revolutionary idea. With its newfangled second derivative motion circuitry that beeped over the top of the target, with a little practice you could tell where the target was. ......Another peculiarity of the 1260-X project was that most beeperists back then were used to hunting in a static (non-motion) mode with a background threshold hum, and when testing prototypes the fact the machine ran quiet they found very unsettling. So we added a fake threshold knob to dial in a background hum (produced in a separate audio circuit) to reassure the user that the thing was still turned on. The subsequent generation of customers accepted the concept of "silent search" so the fake threshold knob was not repeated on later products.
--Dave J.