Have to wonder how they did that ... since the gearing changes the ratio of rotation between the cog and the Hub-shell
(where it would be spoked into the wheel ) As I said, the straight forward method is a 4 bolt cog (normal chainring)

that would be Left hand driven..

I'd have to Guess they used the chainring riveted to the right Hub flange method


Jack shaft ? 'Under the Hood' pictures would be enlightening


edit; & Florian Schlumpf also made a 2 speed Unicycle hub .. 1 & 1.5:1 I believe..

built into a 24" wheel, kicked into over drive you have a 36" wheel ..