RC9YC4
RC12LYC
R = Resistor Core Plug
C = 14m/m thread, 3/4" reach, 5/8" wrench hex (Shell Design)
9/12 = Heat Range (I think 12 is somehwhat hotter, but I don't usually handle Champion plugs. I'll have to C/R to Autolite...)
C4 = "Copper Plus" Design, "Wide Gap" Design
LYC = Skirted Shell, Standard Core Nose, "Copper Plus" design.
I'd not think they'd interchange (going off of that,) but I'd check a couple other parts lines to make sure...
5-90