I thought the whole idea of the middle pocket on the 2540 was to allow a master deck to be read in and fed to the middle pocket. A total card in the deck could be selected into read pocket 1 and a new total card punched and fed to the middle pocket merging it into the master deck in the right place.
A fun reader was the 2501. The card read in 9 edge first then changed direction and passed by the light based reader column 1 first and then shot up into the stacker. The card was stopped by 2 fingers hanging down. Tape the fingers up out of the way, run a program that read cards and you could have a game of 2000 card pickup in 2 minutes. Cleanup was a pain. 😊 https://en.wikipedia.org/wiki/IBM_2501
