It probably depends on the year.
By the late 70's most likely a device called a slitter, which would cut the sheet into strips. the strip I bought of 79s with 78 backs is a vertical strip, and supposedly how these were found at the dump the printer used.
Earlier sets might have been done on a guillotine cutter, which could take a stack of sheets and make the same cut to the entire stack at once. Usually its easier to cut the sheet into blocks then into individual cards since strips are awkward. I think 52s were probably done like this because there are some existing blocks.
The change may have come with the change in size for 57 but I'm not certain.
More current ones are partly die cut.
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