Thread major diameter is 3/4" or 1"? If so, they look like Triumph (and BSA?) fork stem nuts. These screw on to the top of externally-threaded steering stem attached to the bottom yoke, the nuts pass through the top yoke and press on the top steering bearing, allowing adjustment of the preload on the bearings.
IIrc, pre-'69 (circa) the thread is 26 tpi, later is 20 tpi UNEF (Unified Extra Fine).