What are the evenly-spaced "bumps" on Bell X-1 tailplane?
This documentary is about the X-1 breaking the sound barrier. It shows that the bumps are indeed hinges, and that the horizontal tail was indeed originally designed as a trimmable stab and elevator. At 25:40 the control surfaces can be seen moving.

At 35:00 Chuck Yeager talks about losing control at M.94 with this configuration, and then having the tail config reworked into an all flying tail, directly connecting the stabiliser to the stick.

Update Aug 2019

The original clip has been removed, this is the new link. Starts at the movement of the all flying tail.