Hello, Mike,

Here are codes I posted:

"Both of my 261 thermostat housings show the following codes:
GM part number for base: 3764748
GM part number for top: 3764749
Both parts are listed in the 1959 Parts Catalog - 60 series with 6 cylinder engine (261)"

I will check those codes again, now that I have cleaned one of the housings.

Regarding your explanation of the bypass: "As the engine warms up the bypass ports will close and the main port will opened to allow full flow of coolant through the radiator."

That is also my understanding, however, I do not see any "by pass" ports on the thermostat. I did not try the hot water test that you suggested above, but I will try to remember to do it today and I will post a few more photos. (I am regularly getting kicked-out of house and garage for prospective buyers to view our house/garage - each time I have to put things away).

Thanks,

Tim
1954Advance-Design.com
1954 3106 Carryall Suburban
1954 3104 5-window pickup w/Hydra-Matic
The 216 and early 235 engines are not "splash oilers" -- this is a splash oiler.