Awesome find! I'm really happy for you. Survivors are sooo cool. I think you have the correct master cylinder. I had a 70 GTO Judge and restored it with the later, "square corner" type and got knocked in points judging for that, so that's why I think the rounded one you have is correct. But, I'm not a Buick expert. That's a great body style. Probably the best year for the GS. Enjoy it and have fun with it.
2012 Jeep Grand Cherokee Overland
2018 Challenger SRT Demon