Getting back to the original post-- the photo does not show--- but does the machine have a red-colored push
button; If it does is is a classic. I still have mine. Yes they were first available about 1979 and were known as "whip machine" as the faster yow whipped it over a target, the better it worked. Swish, swish--- like playing golf---
Yes--- it has the 1974 patent number, originally the White's Five-Supreme--- THE FIRST Civilian VLF detector!!!! George Payne the engineer, he is still active and designing machines.
The one shown, if it has a red-push button, is the first White's VLF- VLFGEB. The beginning of the the first 6000 D/series.
A collector would likely pay $150 for it.