Thank you to John Medley and Catalina Park for your informative posts.
So it looks as if the Pentecost Nota was built to conform most probably to the F3 500 regulations, and with it being known as the 'Nota AJS', it must of had an AJS 500cc engine in it.
However, the AJS 7R engine only ever appears to have been built as a 350 cc capacity engine, so it must have been another AJS engine type in the chassis?
A quick google search suggests that the only AJS 500 cc motorbikes from this period were the Type 18 and Type 20, spanning the years from 1945 to 1961. Perhaps the Nota was powered by one of these model AJS engines?