The garrett "fortune hunter" TR discriminator predates the "Groundhog" by a few years. You say "1977", for that fortune-hunter, but I'd say more like about '75 or so. And by then, there WAS already ground cancel (all-metal ground cancel at least). Whites had had that (aka "GEB") by 1974. Perhaps Garrett was a little slower to adapt, or buy into the patent, or whatever. The Groundhog dates to about 1978 or '79, and was, as you say, similar in the TR disc. to the fortune hunter. Except that it had VLF all-metal as an option on it.
The both were good machines for their era. Except that by the time the Ground-hog had come out, TR disc. days were ALREADY numbered. Because by then, motion disc, (aka GEB disc) had already hit the scene. And those that got those (Red baron, 6000d, etc...) in 1977 to '78-ish, were quickly spanking those with TR disc. About the only exception might have been in trashy spots. Because the early motion discriminators were extremely fast swing, thus prone to masking. That was perhaps the *only* advantage that a TR disc. might have had. But by the early '80s, the swing speed had been slowed way down, and TR's days were over.
As far as the comparative VLF/TR's, that Groundhog was probably at the top of the game. Like if you were to compare it to the 5000D series I, it was much better.
