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I'm new to Bromptons but I think so. Don't think anything ever comes attached to it on stock bikes so it's a bit weird. I thought maybe it was from bottle dynamos back when they were popular but I think it's simply for the cable rub attachment. Some of them face backwards which might help keep the cable away from the tyre on bikes with no mudguards
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It's on my 2015 6 speed model. Nothing screwed in though, as the boss is only meant to have something there with mudguard-less models, as then the brake cable can interfere with the wheel or something like that.
Here's the modern version of the part that would fit there. I don't remember it being a disc like this. https://www.condorcycles.com/products/brompton-cable-fender-disc?variant=42321712074
EDIT: okay found a pic...
I think only on the drive side if I'm not mistaken. The left side stay goes to the axle with the chain stay catch