Epson EQUITY 386SX/16 PLUS - Manual (Page 113)

4. The math coprocessor socket is located on the front of
the main system board near the SIMM sockets, as shown
below.
math coprocessor socket
5. The math coprocessor socket is hollow and square, and
has a notch in the lower left corner, as shown below.
notch
Installing and Removing Options 5-29