The Manx engine and indeed the whole bike was in the ascendency then but even so the competition were experimenting with multi cylinders, higher revs and it would even then be obvious much work would be needed to keep the ohc unit competitive. Perhaps there was a feeling the rotary valve might just be a way forward to a smoother engine.
How the concept came to the attention of Norton race shop chief Joe Craig is explained by Roy Bacon in his book Norton Singles. Laurie Bond, famous for the Bond Minicar among other things, designed a rotary valve to sit on a JAP speedway engine. Joe Craig saw it, liked it and encouraged Bond to develop the idea at the Norton factory.
As can be seen from the line drawing the rotary valve sat cross way on the barrel, its carburettor on the end of the valve.
An interesting valve drive arrangement was needed to give the valve room to work. As the rotor was driven by the ohc shaft, which wasn’t far enough out, a stepped shaft was devised, using spur gears. The gears also pumped oil up to the massive bearings supporting the rotor. Though power matched the standard ohc unit of the time, there were other problems with it, probably not insurmountable ones but Norton policy changed and the project was shelved.