The Apple III Plus has a DB25 connector for the external disk drive I/O port.
The standard Disk III has a 26-DIP flat connector on the cable. To use the
standard Disk III with an Apple III+, you must acquire a Disk III Cable
Adapter. The service part number for this cable adapter is 655-6101.
Future Disk III drive cables will have a DB25 connector and will need no
adapter.
CAUTION: Before connecting a Disk III to an Apple III+, be sure that pin 13
has been removed from the adapter. The pin can either be clipped off
or pulled out. Later versions of the adapters may have this pin
removed for you.