I had some kind of FSM ( Frequency Selective Voltmeter ), i think Sierra 128, and when i tuned across the NLK VLF freq, the meter pegged. I think i had maybe 2 of these, courtesy of J. N., who got them at the General Telephone auctions of pallets of overage. ( Ah, those auctions ! ) But the 128 was simply to huge for me to keep. One got sold for cheap and the other gave up a bunch of nice circuit boards.