The method of making 1+2/ Rectangles on Garbo is described in 24B(c). The
following practical steps were taken to ensure accuracy. The tape was measured
on a hand counter and positions of the form (1271 n +2) were marked. The second
letter of the tape was put in the Garbo (which deltas backwards) and the print-out
was started and compared with a hand check prepared for the first few characters.
Whenever 1271 characters had been printed and the paper was reset, the tape should
have been on the appropriate mark and this was checked. A hand check for the
last few characters was prepared, and the position of the last character printed
was verified by calculation. Garbo rectangles were only made once.
Different markings of the tape would have been required for a 3+4x/ or 4+5/
Rectangle. These were not made on a routine basis.
A further hand check was applied to rectangles when they were returned to
the H Registrar. From the check sheet prepared by her from the Z tape [see
35c(b)] a hand check for the first entries of each cycle of 1271 was made.
(b) Miles and Garbo (Thurlow) Rectangles.
This method of rectangling is described in 24B(d). The tape was measured
and marked at positions of the form (1271 n +1). Hand checks for letters 1-10,
1271-1281 etc. of the Thurlow tape were prepared. Marks 1-5 on the Z tape were
put in the 5 heads of Miles and the resulting Thurlow tape compared with the hand
check. After it had moved 1271 times the Miles was stopped and it was verified
that the second mark was in the first head, the third in the second etc. The tape
was removed and marks 6-10 put in the 5 heads and so on. The start of each new
stretch of 1271 was compared with the hand check.
Thurlow tapes were made twice and measured to ensure that their length was a
multiple of 1271 before printing. The positions 2,1273 etc. were marked and the
Thurlow tape printed like a Garbo rectangle. The position of the change of depth
was calculated from the Z tape, checked on the Thurlow tape and marked on the
A further hand check, similar to that for Garbo Rectangles, was done by the
H Registrar when a Thurlow Rectangle was returned.
35H OTHER TUNNY JOBS
(a) Hand Perforation.
Hand perforations were most easily checked by printing out the perforated
text and checking the print-out against the original.
The various tapes required for Crib work are described in detail in Ch 27.
(a) Other jobs.
Tunny Room machinery was very adaptable and numerous non-routine jobs were
undertaken. In certain cases it was necessary for hand checks to be prepared by
a cryptographer who (at most) supervised the job in person or (at least)
provided a sheet of careful instructions.