What color is my 1970 duster

And regardless of the paint code, that color will be VERY hard to match. B5 has metal flake in it, and I agree that looks like it could be B5. Although B5 also looks very different depending on the light it's in, partly because of the flake. Also partly because there was more than one formulation for B5, it was different in different model years.

Anyway, touching up any large sections will be very difficult. Even sprayed out of the same bucket colors like B5 can look different if they're sprayed at different temperatures or air pressure settings, because it changes how the flake is distributed. You can't even hang a run and sand it out, the metal flake leaves a "ghost" run in the paint even once it's been sanded flat.

I do not envy anyone given the task of matching that paint.