Epson Equity 286 PLUS - Manual (Page 121)

6. The math coprocessor socket is rectangular. There is a notch
at one end of the socket, as shown below.
notch
There is also a notch on one end of the math coprocessor:
notch
Align the notched side of the coprocessor with the notched
side of its socket. The notched sides must be aligned for the
coprocessor to operate properly, so be sure the alignment is
correct before you proceed to the next step.
Installing
and Removing Options 5-3 I