From Sheldon's site:
In the 1970s, there was a move toward 6-speed freewheels. These were of two types:
"Standard" spaced 6 speeds had sprocket-to-sprocket spacing the same as the existing 5 speeds, around 5.5 mm
Standard spaced 6-speeds required increasing the frame spacing to 126 mm, aggravating the problems introduced with the move to 5-speed, but still providing satisfactory service in most cases.
"Ultra Six ®" spaced 6 speeds used a closer spacing, around 5 mm. This permitted an Ultra Six ® freewheel to directly replace a standard 5-speed unit on a 120 mm hub.............