Hi Stew.
I know nothing about this engine but my initial thought is that the builder has utilised the castings (fabricated component?) that would normally have been a cylinder / valve chest and made valve bore operate as a second cylinder
I can't see how it could operate as a compound unless the low pressure cylinder had a separate valve independent of the rocking motion as at the moment the 2 cylinders are permanently in phase with each other.
It would be interesting to see how the cylinder / portface is machined / drilled, that would tell us a lot