I ran across this black and white pic today on Facebook. It shows one representative model for each GM division in 1959. Cool pic - I like it.
Drilling down on it, clockwise from top-left, Cadillac, Buick, Olds, Pontiac and Chevy.
It appears all 5 cars may share the same front glass. In the rear, the Caddy, Chevy and Pontiac all have swept glass while the Buick and Olds have wrap-around rear windows.
1959 was probably the pinnacle for tail fins. In a few years, they would be gone.
I'm pretty sure the Caddy is a "c-body". For sure, the Chevy and Pontiac are b-bodies because they never offered c-bodies in those brands
The Buick and Olds are more difficult to resolve.
Looking at the color pics below, the LeSabre has a wrap-around rear window while the Electra has swept glass. Conversely, the Olds 98 has wrap-around glass versus swept on the 88.
In the case of Oldsmobile, googling I found 88's and 98's with both type rear windows, so that's not an identifier. Same for the Buicks - I found both models with swept and wrap-around rear windows. So the rear glass doesn't help with the b-versus-c question.
The sources for the pics indicated that the LeSabre and 88 were b-bodies while the Electra and 98 were c-bodies. Yet looking at the color pics, I cannot see anything to differentiate the b's from the c's - the Olds 98 sheet metal looks like it would bolt onto the 88, and the same for the LeSabre/Electra.
There may be other, hidden differentiators, but I wonder if the only difference here between the b's and c's is that GM called one b and the other c, with no other differences
Researching this, looking at other factors such as length, I found that the Electra came in two different lengths, the short one called Electra, and the longer one called Electra 225, with overall length of 225". If you ever wondered the origin of that term, now you know.
One thing is clear, the sheet metal for each brand is decidedly different. It appears that in 59, in their badge engineering, GM was doing a very good job of hiding the similar features of the different brands.