I just assembled and install the seat on my '71 3511. It was totally frozen with rust, took a lot of beating and finally a 20 ton press to push out the pins. All shims and fasteners ruined. The original bushing material looked like brass net with graphite. It is approx. 0.5mm thick. I replaced it with what I had at hand, brass tubing from a sink
Drilled out the pins and added grease nipples, put everything in position and fixed the pins by 6mm bolts and small fins welded to the end of the pins. Not pretty but will surely outlast me