Baube: The number of portholes on a Buick is different for different models for different model years.

Just for my own curiosity I made a small list:

1949:
Three portholes = Super
Four portholes = Roadmaster

1950:
Three = Super or Special
Four = Roadmaster

1951:
Three = Super, Special or Custom Special
Four = Roadmaster

1952:
Three = Super or Special
Four = Roadmaster

1953:
None = Skylark
Three = Super or Special
Four = Roadmaster

1954:
None = Skylark
Three = Super, Century or Special
Four = Roadmaster

1955:
Three = Special
Four = Roadmaster, Century or Super

1956:
Three = Special
Four = Roadmaster, Super or Century

1957:
Three = Special
Four = Roadmaster, Super or Century

The one in the picture is a 1955 with three portholes therefore it can only be a Special. By the shape of the C-pillar we can see that it's a two-door hardtop coupé, known as a '2-Door 6-Passenger Riviera' in Buick language.

A complete identification would read: '1955 Buick Special 2-Door 6-Passenger Riviera model 46R'
A complete identification at IMCDb would look like this: '1955 Buick Special Rivera Coupe [46-R]' but I think it's wrong (Rivera is even a typo...)