"R" prefix plugs also have another important function besides limiting radio frequency interference- - - - -they prevent erosion of the contacts inside the distributor cap and the rotor tip by limiting the current flow across those contacts. A "hot blue spark" that was once the ideal situation is also the equivalent of an arc welder where distributor cap conductors are concerned. Resistor plugs and/or wires prevent damage to critical components while still providing plenty of fire to light off the fuel mixture. "Wire" conductors for ignition systems went obsolete over 50 years ago.
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!