Mark, I would have to respectfully disagree. The timing cover is numbers matching (609) to the rest of the case, which is a Super case (616/12). Please see the excerpt below showing Super cases in 1963 had serial numbers in the 706xxx range.
Bottom line: The timing cover is original to the rest of the case, which is a Super case, and the serial number on the timing cover is right for what this engine is (a 1963 Super, with 75 HP.)
I would appreciate it if you would please correct your statement to remove any doubt you may have caused. Thanks.
