Of the four series of Buicks for 1956, the Special and Century were
the 'lightweights', with the Century being more expensive and more
powerful. The Century (reintroduced by Buick in 1954 after several
years of absence) was essentially a Special with Roadmaster power. It
was considered the fastest Buick. Both Special and Century look
virtually identical, and shared their body shell with Oldsmobile.
The upper two series for Buick were the Super and Roadmaster. Both
were larger and heavier, with the Super being the cheaper and least
powerful of the two - a poor man's Roadmaster. Their shared body shell
was also shared with Cadillac.
Forget about the number designations which are a leftover relics from
an earlier time - they confuse things.
Riviera was a moniker Buick used then for their pillarless hardtop
models. 1956 was actually the second year for the Special and Century
4dr Hardtops, while it was the first year for the Super and Roadmaster
4dr Hardtops, which look and are rarer than the Special/Centuries.
The 1955 Special and Centuries, along with the Oldsmobiles, were the
first pillarless 4 doors offered by Detroit, and caught everybody by
surprise. It was not until 1956 until Ford and Chrysler (as well as
the rest of GM) came out with their 4 door HT models.