Epson 3865X Plus - Manual (Page 137)

4. The math coprocessor socket is hollow and square. The
lower right comer is notched, as shown below.
-notched corner
There is also a slightly notched comer on the math
coprocessor. A small, hollow dot on the top of the
coprocessor marks the notched corner, as shown below.
dot
notched corner
Installing and Removing Options 5-31