The patent might be traceable Hugh?
I'm more than likely reading it wrong, but based on a few mins of staring at it i would guess that 1st & 2nd seem more or less conventional; top is engaged by sliding the 3rd motion shaft rearward from the position drawn; and that 3rd is perhaps achieved by sliding it forwards, so that the internal teeth on the rear gear cluster engage with the external teeth on the 'floating' cog in the middle, which is supported by the extra bearings (which presumably fit into something which isn't drawn). The key point being that the 'floating cog' is not concentric with the shaft. Heaven knows what ratio that would give. I might be very wide of the mark! One thing I note is a bush in place of that horrid intershaft roller.
I'm more than likely reading it wrong, but based on a few mins of staring at it i would guess that 1st & 2nd seem more or less conventional; top is engaged by sliding the 3rd motion shaft rearward from the position drawn; and that 3rd is perhaps achieved by sliding it forwards, so that the internal teeth on the rear gear cluster engage with the external teeth on the 'floating' cog in the middle, which is supported by the extra bearings (which presumably fit into something which isn't drawn). The key point being that the 'floating cog' is not concentric with the shaft. Heaven knows what ratio that would give. I might be very wide of the mark! One thing I note is a bush in place of that horrid intershaft roller.