The B-screw is screwed into the upper knuckle of the derailleur. Under normal circumstances, the chain tension pulls that screw's end against the derailleur hanger's notch - it's free-moving. Later (I think) Shimano designs incorporate a sprung pivot there to allow the guide pulley to better track the cogs a bit.
In a way it's nice, though. With the wheel removed it's easy to move it out of the way by hand. It also simplifies it, and that function doesn't add much value on a friction shifting bike, anyway.