Southwind, great story. Having been into this for 35 yrs. now, I've seen park's with similar-to-that stories too. Ie.: we'd struggle, back in the late 1970s, with our TR disc, to reach the '40's wheaties. And as you say, knowing the age of the park, and seeing the clear stratification by age of the coins (ie.: the older the deeper), it just "stood to reason" that there were older coins, deeper than we were reaching. Then in 1979 and '80-ish, motion discriminators hit the scene (the original 6000d, for instance), and the park "comes alive" again!. And so on, and so forth, with each new technology advancement through the years. And yes, the Explorers seem to punch a little deeper yet, and presto: more silver coins from that next 1/2" of un-charted territory
I'm only using an Exp. II though. I've hunted alongside E-trac users, and we commonly trade off flagged signals. There's no whisper they get, that I also can't equally hear, and vice-versa. So I'm not so sure the Etrac has more depth for park turf silver, than the Exp. II has.
And another note I'll add: some parks definately are "worked out", and I don't care how much deeper new technology will reach, there are simply no more to be had. These would be places where ....... for example ...... picnics were held out in the country, in what is now a cow pasture. I can think of a few places like that, where we combed every last living signal out of, and did, in fact, easily reach down to the oldest coins that could have been expected (ie.: based on the age of usage of the picnic site history). In places like that, where depth or junk wasn't an issue, to begin with, I'd imagine that newer machines won't necessarily open up more to be found there.