Corrosion likely to be found on an old t-stat.
The 427 t stats are a part of the circulating pump housing, and they are set into a seal assembly. The seal assembly is not unlike a main seal on a crankshaft, and it's sort of strange to see how these are used in a water based cooling system. The thermostat pulls directly out, leaving the seal, which is replacable.
If you're careful, you can pull the t-stat out and reuse the seal. If you mess up the seal, then you'll have to get a new one from a hydraulic equipment supplier, power transmission vendor, or bearing and seal sales dealership such as Allied Bearing. I believe the correct number is 16020 as manufactured by SKF. I'll try to verify that one.
The thermostats may be superseded by a new style (as shown above) and I'm in the process of verifying that too. The old number was Rostra Vernatherm Valve # VD111458B (140-degree, better verify).
In the interim, if anyone has any experience obtaining these from parts recyclers, please let us know. Please also help us verify the thermostat temp range, as I believe the t-stats for a closed cooling (heat exchanger) system is 140 and the raw water system is 160.
Regards, Paul