If it's always the same letters just use the Format property of the
field to display it.
But note that Autonumbers will *always* have gaps and can become
random; most developers avoid displaying them in any way that users
will even see, much less need to note.
If the letters might differ from record to record, then it's much
better to store them in TWO fields, a Text field and a Long Integer
field, and concatenate them for display.
