Its been a very long time since I fitted one, but I recall they weren't the best fitting things I've ever used. It looks like the one you want is the M9649, though some people claim it doesn't fit (or doesn't fit well) others claim it does. Oddly the one on the UK Apple store gets a good review while the US version does not.
You might ask them to check the service parts DB to see how many different ones they can find on there. Those would have 922- part numbers.
I know for a fact this part was for sale back in 2008 as I just found it on a price list (Can't be sure if its from June or October due to the date format) but the 30" was still being sold at the time and the 27" was new in October so might just about have been available. Its still available now, just search VESA on the Apple store.