I used some of that Kinder, Friendlier, Muriatic Acid today.
Applied inside the Tube with a Foam brush. I let it sizzle and eat the Park for about 2 minutes then applied some more.
After 4 minutes of total time with the M Acid on it I rinsed it with cold water.
It stripped all the Park from inside and revealed a thin coat of the Original Bluing.
I was careful not to get any on the outside of the Tube.
But after seeing the Original Bluing revealed inside, I would have applied over the outside Tube..... If the Acid hadn't eaten away the foam tip.
It was the only foam brush I had or could find.

When I have some extra time, I'll experiment on some more, maybe by just dunking them and limiting the time the Mur Acid is on it.

Thing is:
I seen no sign of any media blasting on the Park'd metal.
I expected NO sign of Bluing underneath.
How did they Park over Bluing ???
I can't be sure but assume it's Zinc Phosphate, but like I said I can't be sure.