Epson 3865X Plus - Manual (Page 118)

The illustration below shows the four standard option slots
inside your computer.
option card slots
I
1 2 3 4
Slot 1 is designed for an 8-bit option card and slots 2 through 4
are designed for 16-bit cards. As you can see below, a 16-bit
card has a special connector along the bottom.
16-bit option card
8-bit option card
connector
connector
Installing and Removing Options
5-11