I know this is an older thread, but I couldn't help but jump in this kind of historical breakdown is exactly why I love this forum.
You’re spot on with the numbering logic. Buick’s system was actually quite brilliant for its time because once you knew the "code," you could identify exactly what was driving down the street. For example, in 1950, that Series 40 Special was the bread and butter, but that Model 56C (the Super convertible) you mentioned is now a high-dollar collector's dream.
The logic held firm: Series 40 was the Special, Series 50 the Super, Series 60 the Century (when it was in the lineup), and Series 70 the Roadmaster. It stayed remarkably consistent until 1959 when they ditched the numbers for the names we remember from the 60s LeSabre, Invicta, and Electra.
Thanks for laying this out, Ben