AT&T 6300 Programmer's Manual page 115

Disk File Handling
Each time the PUT statement is executed, a
record is written to the file. The two-digit code
that is input in line 30 becomes the record
number.
Note: Do not use a FIELDed string variable in
an INPUT or LET statement. This causes the
pointer for that variable to point into string
space instead of into the random access file
buffer.
4-31