65 Barracuda.....what does this booster do ?

Hi guys.

I'm in the process of getting an old Barracuda, and had no idea what the odd looking black thing was on the inner guard.
After googling a bit, apparently it is something to do with boosting the brakes ??? I've never seen these before - what the heck do they do.....there's already a booster on the brakes.

Just thought someone might know a bit more.

Thanks,
Peter

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