IBM PCjr Manual page 40

Let's take a look at two other keys that may be
confusing because they look so much alike. These are
the lowercase letter "1" key and the number "1" key.
We are going to show you the difference between them.
Press the key with the letter "1" on
it. It is just under the letter "O" key.
Press the number one (1) key. It is
on the top row of the keyboard.
Notice that the lowercase letter "1" is thinner than the
number"!".
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