I talked with John Rotella "the Code Cracker" about this subject. "In the years 1958 through 1976, Thunderbirds shared the same assembly line with Lincolns (as was the case in 2002-2005).
Ford VINs always start with the number 100001. During the SquareBird Era, Lincoln VINs started with 400001. So it is easy to tell how many were built, or to be precise, how many were scheduled to be built.
From what I have read and understood from people who worked on the assembly line, in the SquareBird Era it was most common to see Convertibles only a "Convertible Day", they never ever mixed Hardtop production with the Convertibles. Lincolns had their own separate line on which they would be veered off for some special items, and then the Lincoln Convertibles had their own special line off that one! It must have looked like a railroad yard.
The only place where Thunderbird production is confusing was at Dearborn (1955-1957), and Los Angeles (1968-1976) when Thunderbird production numbers were merged with other Ford products.
All Lincolns were built at Wixom starting in 1958, and continuing right up until the present with these two notable exceptions: the Versailles (built at Wayne with the Granada and Monarch) and the present-day MKX (Oakville).
Right, this photo is correct. As the assembly line process neared its end, the Lincolns would go off to the side for some special quality checks, and tests of some components that the Thunderbird did not have.... like automatic dimmer or power door locks. Then the Lincoln would get back on the regular line. In late 1957, when the new big Lincolns were being built at Wixom but Thunderbird production had not yet started, they really enjoyed some intimate "hand holding". A retired ex-manager from Wixom told me a while back that he felt the reason that Square Bird production was delayed was really to accommodate the workers in a new surrounding, and to get the Lincolns on the right track.
Every road test comparison of the day, Cadillac vs. Lincoln, always puts Lincoln ahead in the quality control department. Not to say that they were such wonderful cars, but for 1958 standards they were the best in the U.S. "