I have been talking with Dane at Sargent about replacement foam for my '73 R75/5. They're getting out of that business, and the employee who understood which foam goes with which old BMW is no longer there. I was basically told they couldn't help me.
Thanks All......As this is not for myself, I have recommended that replacement seat foam be built up in layers, of something like older, foam....backpacking/ sleeping pads. Cut to shape with an electric carving knife.
Siebenrock has /5 SWB seat foam listed on their website. I asked about /5 LWB foam, and was told it's the same as their /6 foam, which should be back in stock in a couple of weeks.
I just restored a /7 seat. The grunt work on the pan I did. The cover was ordered from Motorworks for 26 GBP and I had a local guy sculpture seat foam and attach the cover. All for less than $200. It looks a good as the OEM version.