Mains adaptor with (as best I can measure it) 2.1mm width plug (shown in close-up), outputting 12V/1.0A.
It is brand new and unused; it came with an electronic gadget but was the wrong spec!
It seems to match the spec for some broadband routers (as I found while rummaging around to see if I had an adaptor that would work with my new gadget!), and no doubt other similar things.
Mains adaptor 12V/1.0A output (CB2 near Cambridge Station)
This offer has been gifted to someone and is no longer available.