The "R" prefix indicates "resistor", which was a newer type that the original AC 43, 44, and 45 plugs that did not have an integral resistor in the insulator. Those plugs are virtually collectors' items, and the R series aren't much more common. DO NOT USE a plug with a "TS" suffix- - - -that stands for "tapered seat" and it definitely does not interchange, even if the numbers are right. TS plugs usually have a 5/8" wrench size, rather than the 13/16" flats on a R44, R45, etc. NGK or Champion makes a suitable swap for a Delco plug if the AC isn't available. Just consult a heat range interchange chart.
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!