Basically the difference between the 212 and the 412 is the rotor (plus the side engine covers). Again the kits have the parts with no pitch so looking on the internet I could see that the 412 at rest has no pitch to the rotor (that I could tell ), whereas the 212 rotor did. But looking at the kit parts I couldn't figure out how the rotor worked, so I found a very good article that showed exactly how it did. I didn't have to do very much to it, just figure out where I was going to cut the blades off and then, cut, drill, pin then glue it all back together again.
The 212 rotor mechanism is exposed so the kit has all these lovely parts, the 412 mechanism is mostly hidden behind the mast fairing with only the very top being seen, so there's no parts in the kit or to see.