I have this '1989 Shimano Accushift stem-mounted shifter that converts to friction mode by loosening the black tension bolt a bit, rotating the grey ring to the "Friction" position, then tightening the black bolt again.
The derailer and shifter index correctly with an old 5-speed 14-28 Shimano freewheel (6th click is unused, of course), but chain sometimes slipped in highest gear when I tried it with a 6-speed 14-23 freewheel on a different frame.