For those referring to the old machines with "metal/mineral" knobs, don't confuse all-metal "TR" with "TR disc.". They were 2 different beasts. The TR disc could NOT see through iron. It was only the all-metal TR modes that could do it. Ie.: they would skip over nails (as long as they weren't humongous nails) all day long without making a sound. But if you put a coin under the nail(s), they would give you a signal of a conductive target.
But that was where the benefits ended

They only got 4 or 5" deep max, were a bear to keep balanced, had no form of ground-balancing (simply didn't work in mineralized ground/states), and had no other form of TID (foil and silver dollars all sounded alike).
The last of the all-metal TR machines ended in the mid 1970s, when VLF all-metal became "all the rage". All metal VLF went WAY deeper, and could be tuned to go into any ground, no matter how bad the minerals. Sure, they didn't pass iron, but by then, TR disc. was passing iron just fine, so why bother with all-metal TR anymore? There were very few in those years who recognized the difference between "seeing through" (all-metal TR) and "discriminating out" (TR disc.).
I'll never forget when, in 1980, a lot of us smug jet-set hunter had the latest motion disc, TR-disc, etc... when they tore out the sidewalks of our old-town district. Along comes a guy with a nearly 10 yr. old 77b, with NO discrimination. We dismissed him as a dinasour. Imagine our surprise when he spanked everyone else out there in that nail-riddled environment. I remember asking him "how'd you do it?". He would only say "this 77b doesn't pick up the nails". That made absolutely no sense to me, at the time. I mean, afterall, my discriminator is "passing up" nails too, right? It wasn't till later that I began to see the difference between "seeing through" vs "discriminating out".