I saw this the other day. The SN draws a blank on Everymac, which does mean this isn't just a retail unit given to a dev. Someone else could also check the Leopard build number and firmware revision.
Having said that, the difference between this single penultimate model and the 1.5GHz iteration before it was just a processor speed bump, so this prototype is probably not that interesting internally.
The 1.67GHz launched with Panther and the Leopard Disk (Build 9821) doesn't follow the normal 5 character build number. It was used a long time by the dev if the Leopard disk is his.