So, I've had an idea kicking around for a while that a particularly evil security pin would look a bit like a spool or mushroom pin, but have the middle section filled with a polymer sleeve, possibly cast onto the pin. I would think that a number of different durometer polymers could be used, to make feedback even more difficult.
Much like there are serrated bottom pins, the same idea could be used on at least the longer bottom pins, possibly in the form of a little “hatâ€