So I just stumbled on a New Model .30 Carbine Blackhawk for sale locally. Looked ok, so I bought it.
Getting home, I tried looking up the serial number for year of manufacture, using Ruger's reference here
That reference shows the first year of manufacture of the New Model BH in '74 beginning with s/n 51-02629. Interestingly, my revolver has a s/n of 51-023XX, around 300 lower than the "first" NMBH in that caliber. It's definitely not a 3-screw, and it's marked as:
Ruger .30 Carbine Cal
New Model Blackhawk
So is this a pre-production gun from '73? Something else?
ETA: never mind. I see where Ruger says those serial numbers are approximate and not exact.
Still an interesting find.