Thank you all for those views. My Granddad was in WW1, first in the trenches then in the early airforce as observer in various planes, he'd have been 16 when it started . He survived and went into engineering, civil then working for Armstrong Whitworth building aircraft nr Coventry during WW2. So it's quite possible this was his own handiwork at some period. As for value - academic interest only. I don't have many of his things so I'll be keeping this one.