I was in Jr. High school (7th & 8th grade) in about 1975-76-ish. A fellow student there had a couple of brothers who were 18 to 20 yrs. older than him (they were old enough to be his father! He had nephews that were almost his age, etc....). His older brothers had already served in the military, come home and started families, etc..... The oldest of the brothers had, in about 1962 or so, been leafing through the pages of one of those "Arizona Deserts" or "True West" type of magazines. And in the classified ads in the back, had spotted a little line ad that said "Find Buried Treasure". He sent off for a catalog. And soon, a Whites Co. catalog was in his mail-box. From that he chose a big BFO whites Detector. And by 1963 or so was plying the local school yards around here for loose change. Soon his other brother got into it. At the time, I think they were amongst the first ever to be doing that around here (to anyone's knowledge). They never really found super old stuff, nor expanded to anything very exotic. Just run-of-mill schools was their normal spots. And go figure: Silver was still in circulation. That plus , go figure, minimum wage was probably still only .75c per hour or whatever, so to get a buck an hour in modern change was fun I suppose
By the early 1970s, their youngest brother (my school chum) wanted in on the action. So they got him a Compass 77b (or it might have been a 94b auto), to "keep him occupied". While the older brother swung their big box 66tr's, etc.... Imagine their surprise when that little green box machine out-hunted the older brothers! This was a time when I suppose that the mind-set must have been "the bigger the better", haha
So one day, in about '75 or '76, the school friend invites me along. Went with him to a school built in 1921. I was his "digger boy". I distinctly remember we dug some clad, some wheaties, a buffalo, etc.... I was hooked! I gathered up my entire summer job savings that I'd made picking apples, and plunked down $100 on a used Whites 66TR. That was about the summer of '76. And even by then, that machine was already a dinasour. Guys with discriminators were "kicking our b#tts". So I graduated up to a Garret Groundhog in about 1979 or 1980-ish. But again, that too was, by then, a dinasour, as guys with 6000D's were "kicking my b#tt". And so on and so forth throughout the evolution of the different machines.
I even saw a few BFO's still in use in the mid 1970s. One in particular still in use, the other just more of a curiosity piece. So I can almost say that I "saw the entire" evolution, as those were ground-breaking years to be evolving through the different incarnations.