I have a set of 3 coils 11,8 & 3.5 " that work on my Fisher 660 VLF mother lode detector which operates at 4.5khz. They look like & have the same plug as the 12XX series coils & they operate in the same frequency range. I am looking to buy a used Fisher 12XX series & I would help me make up my mind on which model to buy, If I knew the coils would interchange. I will also be posting this in the modified forum.