This is a Quadlock mount for a Brompton bike. It works well for S-type (flat) handlebars but might not work well for the M-or H-type (riser) handlebar, I don't know.

You may have to trim the little detent points with a razor--I bulked them up a bit to ensure a snug fit on the basis that it's easier to remove material than add it.

The tall version isn't quite as low-profile, but it lets the phone clear my cycling computer.