I'd say you have nailed it Ray.

Dark kero, plus a darkish castor oil.

As a natural product there is seasonal variation to the castor oil. I did hear that veterinarial grade castor oil has a darker tinge to it than pharmaceutical or cosmetic grades. It does seem to be very good oil. I have a dry patch on one hand and it always seems to improve when I rub it with the oily rag after a session!

Kerosene. Is there such a thing as undyed, unadulterated kerosene any more?

There are a lot of things about the engine that could be improved. That said, for what it cost, it does perform well. On mine, one pot has a little SPI going on as well!

The intake venturi does screw in to the crankcase. Just like the CS .15 OT. Next time I run it I will look more closely to see if there is leakage from the thread on the venturi boss or the spraybar mounting holes.

I wonder if Tom Ridley has run his hands over one?