Epson 386/25 - Manual (Page 119)

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