The Metrotech was introduced in 1963, according to what I've gathered on it. It was ahead of it's time. The best for coin-hunting in an era when other machines struggled for 1 or 2" depth. There were a lot of metrotech's doing the dry sand of southern CA beaches (although I don't think they'd work on the wet). And a lot of metrotech's got used in CW sites. By the late 1960s, other makers had cornered the hobbyist market (starting having better products), and Metrotech stopped concentrating on the hobbyist market. They tended instead to deal with the industrial market instead (pipe-finders, etc...), which I think had been their original market anyhow, since the beginning (but hobbyist had just wised up to how good they were).
I've never persnonally used or handled one. But based on what you're saying about it nulling on nails, it might be an all metal TR (not to be confused with a disc. TR). Either that, or it was a BFO (did it have a "motorboat" sound?). Because I think some BFO's had an innate ability to reject iron too.
And if it's an all-metal TR, don't confuse the ability to null or reject nails, with what you know of as regular disc. nowadays. Because sure: discriminators today will EASILY reject iron, right? The difference of the all-metal TR, was that you could put a coin right under a nail or two, and still get the coin (ie.: no masking). But the "trick" seemed to fizzle out by the time you added the 3rd or 4th nail, haha.
Other all-metal TRs were famous for this. Like the 77b, which arrived in about 1971, if I've got my dates right. And so yes: they'd be good in ghost-townsy areas, where a carpet of nails exist, to be able to pluck conductors from beneath the nails.
However, the good news ends there. In all other ways they had drawbacks:
a) they lacked depth.
b) they were poor in minerals.
c) they lacked any other form of disc, other than small iron. Hence everything from foil to silver dollars all sounded the same.
d) there's actually machines on the market now, that do a decent job at seeing through and around iron, WHILE retaining ability in bad ground, and having TID (that's helpful for when the target is not masked, hence not "averaging). Various 2-filter machines that Tesoro makes , and the Whites classic series, are good for this (although not as able as the 77b, or the metrotech you saw). Or try an X-terra with 6" HF coil. A buddy of mine was effortlessly pulling conductors from respectable depths, with handfuls of nails coming out of the same hole.
So there's very little you can't find now, to closely replicate that ability. But from a "vintage" collector's standpoint, yes: the metrotech stands out as one of the "must have" machines of the early 1960s!
