where have you tried to find Sachs,Sram parts , so far?
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2.
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It is mechanically possible, to tear down the hub and add a sleeve of metal in a larger hole
of that internal piece that does the shifting
then re thread it in the smaller thread standard of the remaining SA indicator chain ..

Likely less effort to do as I said above, Buy..
" might as well just get a new Brompton BSR wheel ..

Yes, their new directionally drilled rear rims are better too."

I found the new hub & shifter, Mk4, easier than the AW3/steel trigger in my prior Mk2..





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