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The first Heathkit Grid Dip Meter was the GD-1 and it came on the market in 1951. It was replaced in 1952 by the slightly improved GD-1A, which in turn was replaced by the further improved GD-1B in 1954. The GD-1B was produced until 1960.
The British Heathkit factory in Gloucester produced several kits with the letter U added to the designation. Among them was the GD-1U. The U meant The United Kingdom. The GD-1U differed from the GD-1B in terms of a modernized style but also in the choice of tube and a few other parts. The Brittish GD-1U was marketed until 1969.
For the GD-1U, as well as for the GD-1A and the GD-1B, there was an extra set of coils to buy to extend the range down to 350kHz.
In 1961 the GD-1B was replaced by the Tunnel Dipper HM-10. In 1962 the HM-10 was replaced by the somewhat refined HM-10A. The HM-10A was produced until 1970.
See my HD-1250 for further information.