We never had a Quicktime 7 pro key.
Final Cut Pro 4 had a serial number for the software itself and a key for quicktime 6, which we used.
Final Cut Studio has no such key – the Quicktime panel in system preferences simply reads “Pro Apps User” and enables all the pro functions. The boxes on that panel are empty, however, there is no key to write down. Since we are using Tiger, this means Quicktime 7.
This works fine up to 10.4.5 as far as I can tell. The 10.4.6 update seems to break it.