What are the "spur" devices on this U2 pilot's boots?
These are part of the foot restraint system fitted to the seats used in the SR-71, later U-2 and some Space Shuttles. (They're all derived from the same basic design).

The spurs engage in two ball nipples that protrude from the lower part of the seat. The nipples are connected via cables to reels beneath the rear of the seat which pull the pilots feet in during the ejection sequence.

The seat separation sequence severs these cables (amongst other things) allowing the seat to drop away.

You can see the nipples on the lower part of this Space Shuttle seat:



Source:The Ejection Site