Epson 386/25 - Manual (Page 136)

There is also a notched comer on the math coprocessor, as
shown below.
Align the notched corner of the coprocessor with the
notched corner 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.
5-28 Installing and Removing Options