they are the either teflon or graphite coated "guides" if you may.
The linkage arms had round balls that they swivel on, there should be one of those above and below the ball. If you notice on the end pictured that facing down, there should be slight hollow in the end. Thats where the ball on the arm rubs in. So, those brass nuts on each linkage of each carb holds in a spring, followed by one of those bits, the ball and another one of those bits inverted so that the ball runs in the indents of each "guide".
Hope my explanation helps lol