HP 2566C - Manual (Page 59)

The two labels on the paper tray show you where to position the
backstop for different paper sizes. Refer to Table 4-1 for the correct
placement for your paper size and then install the backstop at the
desired location on the paper tray (Figure 4-2).
2. Position the backstop.
Figure 4-2. Positioning the Backstop
Table 4-1. Recommended Backstop Positions
Form Size (width x length) Single Part Multi-Part
14 g X 12 inches E D
14 1 X 11 inches E E
9 g X 11 inches E E
8 1 x 1 1 inches (3 hole) E E
11 X 8 1 inches G F
8 1 X 7 1 inches I H
4-2 USING THE POWERED PAPER STACKER