They are the same between regular Skylark and GS. There are different varieties out there that look different, but fit and function the same. If you have factory power brakes, the correct master cylinder has a bleeder jet on the pasenger side of the snout on the cylinder. If you get one that has the bleeder on top of the snout, it will work, but is not technically correct for the Buick.
The ones for cars with manual brakes are different than the power. The piston is different to accomodate the rod that connects into the version for manual brakes, and the casting is different in that the manual brake cylinder does not have a bleeder jet on it--only the power brake cylinders have the bleeder on them.