My '86 M46 OD with 240k miles has just special-looking nuts (no washers at all) holding the bridge pieces,

I can't recall seeing these kind of nuts anywhere. I assume they're lock nuts, but no nylon or anything. Ther are about 1/8" thick, but the hex part is just on the lower 1/16" or so. Above the hex is smaller and circular.

The thread is 1/4" x 28. Maybe you can find a locknut that size. Maybe in a bike or motorcycle shop, since I suspect that thread may be 1/4" British Standard Fine (BSF), being a British made OD.
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Bruce Young,
'93 940-NA (current), '80 GLE V8 (Sold), '86 240dl, '85 244 ti, '83 245t
'76 244 (R.I.P at 255k), 73 142 (98K), '71 144 (ex circle track car)
Used '70 144 from '78 to '92, New 144 from '67 to '78, Used '62 122 from '63 to '67