I have found an old photograph of my late Great Aunt Peg standing next to one of late Uncle Frank's motorcycles. I've been told he had owned many bikes over the years. This particular bike is mystery though. I had spoken to my father recently and mentioned that I felt the bike and photograph were likely of 1938-1940 vintage. I ventured this guess based on the girder style front forks. He felt that the photograph was taken while on holiday back in Ireland sometime after the war. That would certainly support your and my father's thoughts.
I do know that both my father and uncle were living and working in England during and after the war. This would also explain the JY registration. One of my previous contacts pointed out that the registration is English and the JY prefix suggests that the bike was registered somewhere in Plymouth.
The mud on the bike would imply he had covered quite a distance. I have done months of painstaking research and believe I have finally found a match with the photographs of a Royal Enfield Model C on this site.
Let's see now... Oh yes, interesting fact about the pannier racks. I had thought for certain that they were something that was bodged on by my Uncle Frank. My first observation of the racks was just how utilitarian they appeared. Very short on style but quite functional. This is the only picture I have of his Royal Enfield but all of the snaps of his subsequent bikes had similar racks. As I mentioned before he was an avid rider and never left home without his foul weather gear in tow.
I now know that they were army pannier racks and not made by Uncle Frank and that the bike will probably be, as you pointed out, a model CO, looking at the cylinder of the OHV 350cc CO and the SV 350cc model C.
As best I know the bike is no longer in the family so I cannot offer any further information on frame and engine numbers. Pity though, it looks like such a great bike. I'd love to have it now.
But at least I know a lot more of the history of the bike on this photograph.