I think that's a reference to thrupenny bits, but it might be a cryptic clue instead.
My F164A went for 120, several years ago (2011) these were well up at 500+, so the market is either shrinking or technology is allowing people to see things more clearly, thus meaning more are being found.
I wouldn't go flogging all your penny collections just yet, fearing they'll be worthless - so many people these days are doing the coin-hunt and collecting 50ps, £2 etc that even if 3% go on to other coins it should be enough to keep the hobby very much alive and well.