Epson 3865X Plus User 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
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Installing and Removing Options