[Dave at Pelican Parts]

For the 74 model year, they eliminated the vacuum advance, but kept vacuum retard. They kept the fitting on the distributor dashpot, but eliminated the fitting on the throttle body. There was still one fitting on the throttle body, and it was hooked up to the fitting on the dashpot that pointed back toward the distributor body.

Sounds like that's what you have.

What are the symptoms you have? The timing should be changing with RPM (up to about 3000 RPM, which is why you run at 3500 when you set the timing) and should also change when the throttle is closed.

When is it "not staying"?

--DD