We agree and the important dimension is therefore the diameter of the flanged copper sleeve which measures 11/16'' on the Robertshaw thermostat.
That's why I measured the flanged sleeve from the one sold by SUMMIT and it also measures 11/16'' (17.46 mm):
So I don't see what law of physics could prevent it from shut off the bypass properly.
It would be all the more surprising since I have just noticed that the packaging bears a label with the SUMMIT reference on one side and on the other side:
The only reason I chose SUMMIT over Flowkooler was because I had something else to buy from them and was saving Flowkooler postage.
So there is no reason to be upset.