Epson EQUITY 3865X/20 PLUS - Manual (Page 141)

3. The math coprocessor socket is hollow and square. The
upper right comer of the socket is notched, as shown below.
notched corner
There is also a notched comer on the math coprocessor, as
shown below.
notched corner
Align the notched comer of the coprocessor with the
notched comer of its socket. The notched comers must be
aligned for the coprocessor to fit into the socket, so be sure
the alignment is correct before you proceed to the next step.
Caution
If you insert the math coprocessor in the wrong position,
you could permanently damage it.
Installing and Removing Options 6-21