The C5AZ 12370 A services both metal line and rubber hose(nipple) as the parts are swapped into the service housing. ( 66/up only,pre 65 used a different part B9AF )I will be re-entering the distributor restoration very soon,but service will be a bit slow. ( I lost 50% of my help )Thanks for kind words. Tim O’Connor
You might want to check your numbers. From what I have, and what I see in Ford Car Parts (aka the MPC), the distributor used on 65 and 66 Mustang 289 "A" code engines (non-smog) was a C5GF-12127-A (as marked, C5GY-12127-A service), which used a C5AZ-12370-A diaphragm.
Jim