5-speed and "standard spaced" 6-speed freewheels and 6-speed cassettes (both intended for 126 mm dropouts) used the same chain width. It's what we would now call a wide chain.
Sun Tour made Ultra-6 6-speed freewheels that were narrower and allowed a 6-speed freewheel to be used in a 5-speed frame/hub (120 mm dropouts). These Ultra-6 freewheels needed a "narrow" chain that is the same width as current 7 and 8-speed chains.