If all three bikes came from the same source when you bought them, maybe at some time they've been switched around to keep 'em running..?
If you're going to have one dated, why not have the other two checked out at the same time; it may shed some light?
Away from yours, I could almost call my Plunger a Super Rocket too, as it has a 67-1126 head, 357 cam Big fin barrels, LJ crank and a 389 monobloc....only the cases are Plunger- stamped 23.12.52. Frame ZA7 19199 seems to align with ~'53
I sometimes consider having them despatch checked, but only for curiousity, I bought everything separate but mostly locally, but I don't expect anything to match. I just want something I can ride.......