The AC 45 is a "hot" plug that was intended for short trips at low speeds like in town or delivery vehicles that saw limited highway driving. 43 and 44 plugs dissipate heat from the tip to the thread area a little faster, which makes the tip less likely to erode from overheating on a long haul. It's a compromise between running a plug tip hot enough to keep deposits burned off, and cool enough not to erode the electrodes or blister the porcelain insulator.
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!