Epson ACULASER M8000 series - Manual (Page 111)

7. Secure the optional paper cassette units with the four fittings. Install two of the four fittings inside
on both sides of the optional paper cassette unit and the other two fittings outside on both sides.
Note:
If you install three optional paper cassette units, repeat steps 6 and 7.
EPSON AcuLaser M8000 Series User's Guide
Installing Options 111