Accordion shaped rubber seal.
https://www.classicindustries.com/p...HxYAgDTeDCdCFbRMSJhoCbxUQAvD_BwE&gclsrc=aw.ds
There is also a part that mounts to the mc
MOPAR MASTER CYLINDER BOOT WITH RETAINER
1965-1966 CHRYSLER DODGE PLYMOUTH *NEW* MASTER CYLINDER REPAIR KIT K-46377 | eBay
BTW
The chrome mc in your photos is not drilled for the bellows / piston stop.
That suggests it has an internal snap ring that acts as the piston stop.
You could drill and tap the mc for the bellows keeper if you feel the need
https://www.classicindustries.com/p...HxYAgDTeDCdCFbRMSJhoCbxUQAvD_BwE&gclsrc=aw.ds
There is also a part that mounts to the mc
MOPAR MASTER CYLINDER BOOT WITH RETAINER
1965-1966 CHRYSLER DODGE PLYMOUTH *NEW* MASTER CYLINDER REPAIR KIT K-46377 | eBay
BTW
The chrome mc in your photos is not drilled for the bellows / piston stop.
That suggests it has an internal snap ring that acts as the piston stop.
You could drill and tap the mc for the bellows keeper if you feel the need