The alternate tooth freewheel was developed by Shimano in the very ealry 1970s to improve shifting with very low gears.

At the beginning of the 1970s bicycle boom, your typical European bicycle used spec'd with gearing that was too tall for the average American, who was new to cycling and relatively unfit. At the time triple cranks were rare and expensive so the Japanese looked for other ways to provide wider gearing as an inroad to the American market. The most cost effective way was to use a double crankset with a smaller bolt circle, allowing a smaller chainring and fitting a freewheel with larger cogs on the back. standard cog designs were adequate up to about 28T but eliminating every other tooth provide noticeable benefits on bigger cogs.

They were generally called alternate tooth freewheels to differentiate them from the old, 1" pitch, skip tooth design as the two designs had different tooth profiles and were incompatible.

The design started to fall out of favour in the very late 1970s with the intoructionother shifting improvements, specifically the Uniglide chains and freewheels.

There may be a two letter date code on the outer cone of your freewheel that would allow you to date it precisely. Note how the 2nd smallest cog has three pairs of normally spaced teeth, to achieve an odd numbered tooth count.