Epson 3865X Plus - Manual (Page 228)

The interface that protrudes from the back of the drive has
gold contacts on both sides. Near one end of the interface,
there is a gap designed to accommodate the plastic divider
on the cable connector.
ribbon cable
connector
drive interface
Align the cable connector with the drive interface so that
the plastic divider on the connector lines up with the gap in
the drive interface, as shown below.
Installing and Removing Disk Drives B-37