I thought the novarossi plugs were rated in ranges from 5tf, 6tf, 7tf,8tf (cold plugs) and 5tc ,6tc, 7tc, and 8tc (hot plugs)

In Italian, F is for frio (cold), C is caliente (hot). I learned that bit of Italian from motorman's website : http://www.richeyracingengines.com/tuning.htm

There's some other manufacture's plugs mentioned there too.