See here for pictures of the tape punching device.
Format: 8 hole positions with, a small hole (presumably a sprocket hole used by the tape reader for advancing the tape), separating the 3rd and 4th hole positions, or 5th and 7th, depending on which way you look at the tape.
From the keypad it seems that 6 of these hole positions, numbered 1 to 6, were used and each key had a unique code depending on those hole positions that were punched out. Thus the send to symbol has a code 2456, representing these and only these hole positions being punched. Similarly, if holes 1456 were punched, and only those, that was a 7. If holes 146 were punched, and only those, that was a 5.
The numbered keys on the tape punch were used to enter the these codes. The keyboard layout below it was presumably not usable and was there to let the user know what digits to enter for a given key. Thus, to enter an A for example, you had to press the 1 and the 2. I'm guessing that keys could be pressed in sequence, and stayed in the down position until the enter key (at right?) was pressed or until cleared (using the lever at left)? Punching paper tape must have been even more laborious than I remembered!
As to holes 7 and 8, they must have had some other function than for defining the keys on this calculator. Perhaps other calculators made by Diehl used these extra holes and the same basic punch device, albeit with different conversion labels, was used by all?
Calculator keys numbered 0 to 9 comprised a 4 character binary representation of the number, prefixed by '10':
key binary code (holes punched)
0 100000 1
1 100001 16
2 100010 15
...
9 101001 136
Most other calculator keys have codes in the form 12...., ie 11xxxx binary, with the exception of the ones shown below. I can discern no patterns with any of these and the allocation seems random.
key binary code (holes punched)
send 010111 2456
, 101110 1345
J 010100 24
P 010011 256