The "bypass" thermostat for the 261 hasn't been manufactured in many years. It had a flapper built into the moving part of the stat that closed off a big recirculating hose that allowed coolant to circulate inside the block and cylinder head while the engine was warming up. That system was only used on medium and heavy duty applications, IMHO. Any thermostat you can buy today will not have the bypass feature built in, no matter what the advertising copy has to say.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!