I don't think, as vern alludes to, that these heater splined arms can be bought, that's why when you live in the most isolated capital city in the world like perth western australia, you are obliged to buy another p6 as a spares car. Luckily I can oblige with a splined arm if you wish off my spare heater as I've just refurbished my original a while ago so didn't need any off the spare heater. Send me your address by private message if you like ('pm' me, as they say on this forum) and I'll pop one in the post.
Scott