Master Cylinder Piston Retention Clip

Picture above shows the stop and boot
FYI: Part you are requesting is called a "stop" 1828 389
Then the "boot" 1828 388 goes on the stop and on the push rod.
Spring 2260 710 goes over the boot between the stop and the seat on the push rod
The seat 2260 709 clips into the groove on the pushrod
Part numbers are from the 65 parts book manual brakes