UNIT Codebook (reference book)

In the early issues of Doctor Who Magazine, a UNIT club page advertised the UNIT Codebook, with an accompanying "UNIT ID", which could be obtained by mailing off a voucher to a specific address. The codebook would obtain two cypher codes, labeled Red and Green. In future issues of the magazine, coded messages would be published which could only be cracked with the codebook.

However, neither the book nor the ID have been officially documented or rereleased by the BBC. Photographs or scans of the codebook are not known to exist due to the rarity of the items, so they are currently lost media.

Continuity

 * The UNIT ID and codebook were meant to exist in-universe, and so, in The Companion Chronicles audio drama Ghost in the Machine, the Third Doctor is shown to carry both in his pockets.