The bellows would be normally attached to a metal retainer.
There is also a plastic one for master cylinders that have an internal snap ring to keep the piston in the bore.
The round ring with the hole in the center is the keeper for the push rod. I believe it is supposed to go into the piston before the push rod goes in, but I'm not 100 % sure about that.
If you don't use the bellows leave the keeper on the MC the way it came, but I think it is only intended to be a shipping part.
If that keeper comes loose and rotated, the piston could come out and you would have brake fluid all over your floor and you would loose your brakes.
I’m going to order the part,thanks for letting me know something was missing. it wasn’t on the old master I just took off. There wasn’t even a dust boot. Funny that they wouldn’t just include that piece in with the new master cylinder.
I’ve seen that video before. Those people are idiots,the could have killed someone screwing around like that. You’re right. Brakes are the most important thing. Especially if you’re hopping up a car like that.