The R means its a resistor plug which, given your (I won't say obsessive ) concern for radio interference is a good thing.

its a chain saw plug

Here's the cross reference to other plugs and I don't see the CM6 listed but hey, throw it in there and see if it runs, your motor won't explode.

http://www.sparkplug-crossreference....BOSCH_PN/WSR6F