[Spooks] regarding NS preables

[EI0DB]

The 3 digit groups sent at the start of messages is more likely 
indicating which pad-set (if using one-time-pads), apage number in a 
code book or even an ordinary book (the Bible is a classic for this) 
used in some super-encryption system.   Since the object of encryption 
is to deprive the eavesdroppers of getting useful information from the 
encrypt-message as transmitted by way of "traffic analysis" using the 
numbers for agent call up would be too obvious.

best regards, Dave