It's where the cable roller bolts on, just to change the direction of the inner cable without too much friction. If you're wondering about the weird threaded hex-shaped braze-on under the chainstay, that's the derailleur mount. All Bromptons have the mount but not many have derailleurs!
If that's a titanium rear triangle, the one thing you *don't* have is a pump peg and clip. Brompton claim this is to save weight (a non-trivial percentage of the weight saving on the Ti bikes comes from the absence of a pump*) but I reckon they just forgot to spec it when they ordered the rear triangles from the Russian factory...
*which is obviously silly unless your bicycle repair kit consists of the Uber app and a bank card
