What spark plugs in my TR6

["Michael Cline" <mcline@substrates.com>]

My TR6 is a 1971 with a non original engine. The engine serial number is
CC82348UE. (I think the CC engines are early and the CF engines are later.)
The ROM specs 9YC for the early engines and 12YC for the later ones. VB
sells only one plug for all years. My question is what spark plug do I use?
What is the difference between the 9 and 12? What are the engine performance
differences?

Thanks,

Michael Cline
71 TR6, 73 Stag, 81 TR7
mcline@substrates.com