He did indeed part with that bass this past year... a friend of a Club Member here bought it, and reportedly enjoys it very much. I pondered it myself, but opted to wait for another that felt more right. Will's recollection is that he ordered his Custom in September of 1975, and took delivery of AC 76-405 in June of 1976. If the bass in the picture is in fact his, (the first fret inlay is the biggest giveaway) there's a good guess to be made based on what stage it was in production.
* I wonder where all those basses are now?