David if you have the IPL (Interim Parts List) for your 1968 (in the JCNA Shoppe) and a copy of the J37 (series 1 4.2 E type parts catalog) all mystery is removed The IPL is a simple list of changes from the 1967 to the 1968 model.
Suggestions that assembly at Browns Lane was random is a myth. Dealers had to maintain and repair cars and would have revolted if this was the case. That and any 30 min read about British Unions at the time would remove that thought.
Lastly the above mentioned head could have been gold! But only if special ordered that way or a dealer did it. If special ordered it would probably be noted on the certificate. Otherwise it was blond alu. like everything else.