Not all card punchers could do that. What we had is a number on the
card that would be the same as the line number on the source printout.
You would *never* have to actually read the card to understand what the
code is doing (which is where the readability is important). You would
only need to read the card to make sure it's the right one just before
replacing it with the updated, in the stack.
By the time the late 60s rolled around, almost all keypunches had, at
least as an option, an "interpret" feature which did the printing. In
the IBM world, you had to go back to the '024 keypunch if you wanted
one that didn't print (its more popular, and otherwise identical,
sibling the '026, did print ). The ‘024 and ‘026 date back to 1949,
and were replaced by the ‘029, which did print, in 1964.
Not to say that I didn’t see more than a few keypunches with broken
printers.
The most common cases where I saw uninterpreted decks was after a deck
was punched from the mainframe, either from a library, or a
duplication of a worn out deck, on the "big" card punch, which
invariable didn't have such a feature. And those were always more of
a pain to deal with because the cards were that much harder to read.
For the life of me I can’t remember which device it was, but there was
a keypunch that had an “interpret existing deck” feature, which you
could feed a deck through it and get it interpreted. Only saw them in
a couple of shops, but they were rather handy at times.