Leon,
The details of the TW tank are from the factory drawing. It has a sloped top like the EW tank, filler at front RH corner, small oil tank compartment at the middle traversed by the hand pump, and between the two caps a tunnel for an oil sight glass.
Other than a few drawings for the TW, and references to it by Jeff Clew in his book, I have never seen a period picture or catalog illustration declaring a TW model. So I do not know what the overall appearance was, other than very similar to the RA which it undoubtedly shared many components with. I have seen three RA frames with the slight slope to the top frame tube. Two has the FF serial number prefix of the TW model, and one had a DF prefix of the conventional RA.
The only petrol tank that I have seen that might have been a TW, was the picture Howard posted in this thread. Of course the factory, or a subsequent owner, could have fitted a standard RA petrol tank to a TW. Doing the reverse would not have been so easy! This would have left the issue of fitting the RA reservoir (you are right, not a sump as the oil never returns to it) or some alternate means of carrying the oil.
-Doug