The following is from Edward Jay Epstein's COUNTERPLOT, pp. 187-189. 
It details one of Jim Garrison's most impressive talents.  He was a
noted cryptographer! 


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A discovery that Jones Harris made while we were going through the
papers provided considerable insight into the nature of Garrison's
investigation. What Harris found was a five-digit number that was
common to both Shaw's and Oswald's address books. The entry in Shaw's
book was "Lee Odom, PO Box 19106, Dallas, Tex." In Oswald's book, the
number 19106 was preceded by the Cyrillic letters [DD]; (which, like
other Russian letters on the page, the Warren Commission had assumed
were made during Oswald's two-and-a-half-year stay in the Soviet
Union). Though the coincidence of numbers proved nothing in itself, it
was striking, and Garrison decided that further investigation was
merited. Shortly thereafter, Garrison announced to the press that he
had found the entry "PO 19106" in both Oswald's and Shaw's address
books, and that the number was a "nonexistent or fictional number,"
which removed "the possibility of coincidence." 

Moreover, Garrison said that "PO 19106" was a code that, when
deciphered, produced Jack Ruby's unlisted telephone number, WH 1-5601,
and "no other number on earth." The method by which Garrison
"deciphered" the code is worth following. Starting with the
"scrambled" number 19106, Garrison "unscrambled" it (by choosing the
nearest digit, then the farthest, then the next nearest, etc.) to
produce the number 16901. Ruby's number was 15601, so by unscrambling
the digits Garrison managed to match the last two digits in the two
numbers. The next step was to subtract the arbitrary number 1300 from
16901, and presto 15601. Finally, Garrison converted the prefix "PO"
to "WH" by a system that, according to the prominent cryptographer
Irwin Mann, yields at least six different prefixes: Garrison chose
Ruby's. 

A few days after Garrison announced that he had deciphered the code,
it became known that the number 19106 in Shaw's address book was by no
means "nonexistent or fictional." PO Box 19106 had been, as Shaw's
address book indicated, the address in Dallas of a man named Lee Odom.
Odom stated that he had been introduced to Shaw in 1966 by the manager
of the Roosevelt Hotel in New Orleans, and had briefly discussed with
Shaw the possibility of bringing bloodless bullfights to New Orleans;
he had left his business address--PO Box 19106, Dallas, Texas--with
Shaw. In fact, Odom's post-office box could not possibly have been the
number in Oswald's book, because the post office-box number 19106 did
not exist in Dallas before it was assigned to Odom, in 1965--long
after Oswald's death, in 1963. 

It was clear that Garrison had done some questionable interpolating of
his own in moving from a coincidence to a conspiracy. First, he had
told newsmen that the number in Oswald's book was PO 19106, although
in fact it was [DD] 19106. (When a television interviewer later asked
him how he had determined that the prefix was PO, rather than [DD],
he answered, with perfect aplomb, "More or less by looking at it.")
Then, without first checking with the Dallas post office whether in
fact such an address existed, he had announced that the post
office-box number was fictional. And, finally, he had converted the
number in Shaw's book into Jack Ruby's phone number by rearranging the
digits, subtracting an arbitrary number, and changing the letters "PO"
to "WH." 

Garrison had constructed a piece of evidence against Clay Shaw and had
disclosed it to the press. Yet the District Attorney did not seem
particularly perturbed when questions were raised about the logic of
his deductions. When he was asked on a local television show how the
number of a post-office box that didn't exist until 1965 could have
been used to represent Jack Ruby's phone number in 1963, he replied,
"Well, that's a problem for you to think over, because you obviously
missed the point." 

Indeed, Garrison counterattacked in a press conference, saying, "We
are very interested in knowing who introduced Mr. Odom to Mr. Shaw,
how many bullfights Mr. Odom has actually produced"--as if this fact
were relevant to his investigation-- and, "We are particularly
interested in clarifying now why there is also coded in Lee Oswald's
address book the local phone number of the Central Intelligence
Agency." Using an entirely different system of decipherment --
multiplying the number by 10, rearranging the digits, subtracting
1700, and remultiplying--Garrison managed to convert the number 1147,
which appeared in Oswald's book, to 522-8874, the CIA's phone number.
Oswald's codes were "subjective," Garrison said, in that they varied
from number to number. There seemed little point in Oswald's having
gone through such an elaborate procedure, however, because the CIA
number that Garrison referred to was--and is--listed in the New
Orleans telephone book.