Came across these the other day while looking at differential bushings:
https://www.z1motorsports.com/transm...r-p-12015.html
The principle is simple enough. Rather than drilling out old bushings to fit poly/solid bushings, fill the gaps in the original rubber pieces to prevent less movement and offer more feedback. At first blush its deceptively obvious - if this solution actually works, why didn't anyone develop it before?