I heard of 'Rom Card locators' and 'Long range locators' that can detect treasure with different Antennas. Secondly, You will never be able to locate their manufacturer, there is no address written on the warranty cards, except the dealer's address. You will not even know, if they are made in U.S.A. or China or Canada. That is how their 'business' work. Gold nugget detectors usually operate in between 18 Khz to 71 Khz range.
Many people think that the higher frequency is better to have.
But it all depends on the soil conditions. 18 Khz to 90 Khz by changing a Crystal or a Capacitor or an inductor. I personally like 20 Khz range detectors for Gold nuggets. Because, If we go higher than that machine would be so sensitive that will die in black sand. Dealers, usually like to sell 40- 50 Khz detectors because they show a very good air test.