Epson Equity 320SX PLUS - Manual (Page 125)

The math coprocessor also has a notch in one corner,
marked by a dot, as shown below.
notch
Align the notched corner of the coprocessor with the
notched corner of its socket. The notched sides must be
aligned for the coprocessor to operate properly, so be sure
its position is correct before you go to the next step.
5. Gently push the coprocessor into the socket, pressing
evenly on all sides, as shown below.
Installing and Removing Options 6-23