Interesting Googling. I put in "np3 28-2" and found this: https://www.gunsamerica.com/amp/972642604/S-W-28-2.htm
Of course, one doesn't know from the ad whether it was really NP3 applied by Robar, or an imitation.
It must have been a severely rusty gun before the NP3. Deep pits that were covered by the coating but led to adhesion failures.
Also, it's completely blowing off the stopping side of the cylinder notches. A 27 or 28 is known for hammering its notches extra severely due to the rotational inertia of the cylinder, but still, that would concern me with any revolver that's going to see a lot of DA.
Hope TGS' project holds up better.
(I imagine a similar result could occur for any spray-n-bake ceramic coating applied to a rusty gun, or in the vicinity of the cylinder notches.)