Pictured is the rear of the VT-278 with RX278 Floppy Drive Interface Connector (drive cable) attached. It's called a RX278 Floppy Drive Interface Connector cable, part number is RC26N-2L. Click image for larger view.
This weekend at the System Source computer museum I used their DECMATE VT278 to build a copy of the OS/8 boot disk for my system. Now that I have a known-working OS/8 disk, I need the cable. I checked page 23 of the ED-VT278-HR-001 Decmate System Architecture Manual for a pin-out to confirm that I don't have the correct cable on hand.