Pics of the reverse side of my 9204 booster from my 01C X77. I removed this from the car around 1980 when I disassembled the car, never touched since. Note there is no evidence of any gasketing on the back of the mounting flanges, only the suggestion of the holes in the firewall where they overlap the holes in the booster flanges. Nuts and split washers (lock washers) are the only fasteners on the front side of the brackets, unplated or maybe phosphate (grey) finish. Note also that there is a thick white felt seal just inside of the plastic sealing tube, secured with a clip, that would keep anything from getting inside of the tube (somewhat) after the tube is located in the firewall hole/opening. The plating of the booster is very visible, and even matte gold color, unlike some of the brighter zinc finishes I've seen on restored cars.
Regards,
Steve