Its a Dave Johnson design. I wasn't to impressed when it first came out but I tried a late model not long ago and it was keeping up with an Eric Foster Deepstar (Mk 1) so way deeper than the Whites or Headhunter P.I.'s.
Claim to fame is its low battery use giving exceptional running time from a handful of AA batteries.
It was originally to be a discriminating P.I. (none motion) but Fisher didn't run with that wanting a motion machine. Bad mistake.
Drawback is the repair situation and it really needs an autotune as you need three hands to use the present design (one for spade or scoop, one to hold the detector and the third to retune it.
Brian