'65 Power Brake Booster Question

In the rear of the power booster I can spot a quarter sized flap of loose rubber around the pushrod. I believe it broke off of the main portion of some kind of rubbery seal. Is this seal important or just a dust cover of some kind?

To replace it, is this available in the aftermarket, can we adapt another rubber thingy, fabricate something, or is it maybe included in a rebuild kit? As I understand it, the '59-'60 Cadillac models use the same booster.

Sorry for all the fotos (the second foto is the best), it's really difficult to see back inside there. I remember having one heck of a time removing/replacing it when it was rebuilt some years back. Thank you.

power brake rubber 1.JPG

power brake rubber 2.JPG

power brake rubber 3.JPG

power brake rubber 4.JPG

power brake rubber 5.JPG

underhood wax 3.JPG