Comment by aap_

The digits and letters actually map quite nicely to punch cards. You can see how punches 0 - 9 map exaclty to EBCDIC F0-F9. And if you check how the letters are coded on punch cards (1 - 9 plus one punch "above" in the zone and 0 rows) you can see how it maps exactly to EBCDIC C1-C9, D1-D9, E2-E9. Most other characters aren't coded quite as neatly, I don't know if there is a system to them.