British-made M-73 compass and pace (tally) counter. These items were used by MFC's in Vietnam to help calculate 'dead reckoning' – a navigation process of calculating a current position by using previously determined positions or 'fixes', and advancing that position based on estimated speed, elapsed time and course.
‘Dead reckoning' in Vietnam required the troops to march on a compass bearing from a known point and to record the number of paces taken between stops using a pace counter. At a tactical stop groups of soldiers would mutually review the distance, bearing, and nature of the ground covered to arrive at a reasonably accurate consensus.
Read more about the challenges of working as MFC in Vietnam here.
Images courtesy Bruce Young