Thanks Eddie,
It certainly could be an experimental engine as Frank's brother, Edgar Meller, who won the 1914 Australian Grand Prix, had only picked up his TT Douglas, which was practically new, from Williams Bros in Sydney 9 days before the race and he advertised that, from then on, he would only be racing Douglas bikes. He said this TT Douglas was just as fast as Frank's Douglas, so having said this about Frank's Douglas, it does give the impression it might have been special.
Frank's grandson was of the impression when I contacted him that Frank had raced for Douglas, but just recently his 93 year old mother who suffers dementia said, "No", he only raced Douglas motorcycles, however she was not born when Frank raced. So from all that Frank may or may not have raced for Douglas.
Peter