All:
This just happened to my '60 Invicta (If interested,see my post on heater box disassembly on this site.) IMHO, that heater box removal was anything but easy; but then this was my first time!
I bought a NOS non-Ranco valve (Everhot) on Ebay. Before I put it back in the car, I had my radiator guy test the valve in place with the core. All okay. For now,at least....
Guy at Smitty's (Landover, Md.) said he can rebuild some valves, but he said he could not rebuild this Harrison, because he couldn't get the plunger.
Well. if this one goes prematurely ,at least I'm an old hand at heater box removal!
CD