Mike, I had the same questions because the NAPA thermostat works like the 16 hole bypass one, both of which work different than Tim's. I can't figure out why some years had a true bypass and some didn't.
Here is another trivia question...why did GM eliminate the hole for the screw that secures the fuel and vacuum lines on the 261 housing? The lines pass in front of the valve cover just like on a 235.
One bit of good news from all this discussion...if you have a '62 to '64 truck with a 261, then the NAPA THM 155 thermostat is available and should be a good replacement for you!
