AT&T 6300 Programmer's Manual page 36

Getting Started
Character Insertion
If you accidentally omit characters in the line
you are entering, then proceed as follows:
1 You notice the error.
Suppose you entered:
10 FO
2-18
instead of:
10 FOR K=1 TO_
2 Use Cursor-Left, or other cursor movement keys, to move
the cursor to the appropriate position:
10 FO_ K=1 TO
3 Press INS and type the letter R:
10 FOR.K=1 TO
Note that, entering Insert Mode, the cursor becomes a half-
height block.
4 Press INS again to return to Overwrite Mode and Cursor-
Right or END to move the cursor to the end of the line:
10 FOR K=1 TO_