I remember that one! It was a slightly hot-rodded VLF-441 with an improved DD searchcoil, I think we still use that searchcoil tooling. Introduced I think 1982. It was basically a Louis Ciuffo (production manager) and Jim Lewellen (CEO) project with little involvement on my part, I was too busy developing really new stuff to get dragged into that one. Irwin Lee was an almost-local (Turlock) dealer who sold to the California central mother lode market and was a gold prospector himself, so he was a logical choice to snag for field testing. Unfortunately for the Fisher 660, the Garrett A2B was the better gold machine until we intro'd the "original" Gold Bug about 1986. That Gold Bug was the first metal detector designed from the ground up as a gold prospecting machine: all prior "gold machines" were adaptations of general purpose platforms.
The 441 was designed by a contract engineer Jim Jones (no, not the guy of Guyana fame, nor related to our engineer Marvin Jones) about 1974, in the early days of ground balanced VLF when the high frequency TR's were still popular and not even BFO's had quite been killed off yet. Very good (and clever) design for its time. Jack Gifford adapted it for the late 70's Fisher VLF-555 discriminator, that was before he started Tesoro. The 441 still lives on in our industrial metal detector product lineup, over 40 years later! ..........Jim Jones visited us in Los Banos about 1982, as I recall an Aussie (or maybe British) about 6'4" tall, very smart fellow. I often wondered what other design work he did, he wasn't a permanent fixture in the beep industry like some of the rest of us turned out to be.