Nazi encrypted teleprinter messages tended to be lengthy, typically several thousand characters. Since the British were intercepting the messages as transmitted from far away by radio, receiving errors were common. So for important Nazi communications links (for example, from the western front field headquarters to Hitler's headquarters), messages were recorded at multiple intercept sites. When multiple versions of the same message reached Bletchley Park, great care was devoted to merging them into a paper tape that most likely was accurate.
The tape was spliced into a continuous loop. The tape loop was put on these wheels, which were then adjusted for the length of the loop. There are two paper tape readers. While Colossus is operating upon one Nazi message tape, the tape for the next message can be put into the idle tape handler. In this photo the tape on the right is idle (the punched holes can be seen) while the tape on the left is a blur because it is moving very fast.
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