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It sucks that Staton changes his site so often that links never point to the intended target for longer than a few months.They're thread-on freewheel sprockets. An example of a shimano freewheel sprocket is shown below. They're designed to thread on 1-3/8 inch, 24 TPI hub. Staton makes several freewheel adapters, to allow you to mount a freewheel to a shaft. His adapters can be used on shaft sizes ranging from 1/2 inch through 1 inch. Available sprocket sizes for this thread-size range from 16 teeth, up through 22 teeth, and I believe I remember seeing a few 25 tooth ones. There are also similar, thread-on freewheels with a flange, so you can bolt larger sprockets up tp them.
Here's a link to his 3/4 inch adapter.
There's another, me(ric standard thread-on freewheel, which threads onto 30mm X 1mm thread spacing, which allows smaller sprocket sizes. (as few as 13 teeth)