Ordnance Survey of Plymouth: [Plymouth fortress defence scheme map]: T.S.G.S. no. 1846.
Southampton :  Ordnance Survey Office, 
1909.
"25/7/41".
"Secret".
De-classified 22-3-85.
Reference: Jewitt. Maps for Empire.
Revised in 1905. Corrections in 1908.
Wanting sheet 2.