Even easier, just go for the "Serial Numbers" link on the top of the page, select BSA and then 1966. That will take you to the 1966 section of Morgan's rather nicely laid out list.

BSA wasn't necessarily consistent with engine and frame numbering, but A65T was the Thunderbolt in 1966, the year letter was added in 1967, then another letter for month in 1969.

BTW, the serial number list says the frame number was A50C in 1966, but that was just for the start of the model year. BSA switched to matching engine and frame numbers during the model year. There's a note about that above the 1966 section, but it looks a little like it is the end of the 1965 section.

and, yes, it does look nice.