When I fist saw the diagonal construction of the valve chest, I gave it some thought and concluded that it was done to allow access to the valve face for finishing with a shaper or hand scraper. It effectively removes one wall of the valve chest. If all the walls of the valve chest were cast integral with the cylinder, I can't see how the valve face would have been finished with the tools of that era.
It also reduces the number of castings. Usually the valve face is cast with the cylinder and then a rectangular box and a separate valve cover is added, more bolts, and more assembly.
This is just an opinion with no supporting data, but after seeing it, I wondered why more engines weren't made that way.
Jerry