Here is a question you might be able to answer..I got my Vivi Nova set and 3 extra heads, short of busting out my multimeter, how the hell does one determine the resistance of each of the heads. They were all thrown in together (except for the ones from the set) and no markings that I could tell..
On the head itself, just above the little ledge above the threads, it should be etched on there. It's pretty hard to see but it's there. My advice though, bust out the multimeter. Most of the 2.4 ohm heads I've used so far have been off quite a bit, in fact I think I've only tried one that actually metered at 2.4, most are 2.8 - 3.2. My APV has a function to measure that so it's not a big deal, I can adjust accordingly.