The only reference I can find regarding this codebook is on page 196 of Ralph Bennett's 'Behind the Battle' (Sinclair-Stevenson, London, 1994): ' . . . but when on 10 June  the Admiralty at last replaced Naval Cipher No. 3 with No. 5, which proved quite secure, it was plain that the U-boats could never regain their former authority.'
Due to the size of these files, (32kB typ, 78kB max) they have been split into 6 groups. All except for the cover were scanned in black and white to reduce bandwidth. However, due to ageing of the paper, editing and ink smudges, quality is not ideal. Obvious blemishes have been edited out.
Front cover, title page and contents pages
Instructions for use
Dates and numbers
Comments welcome: Keith
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