Sorry Mike, missed your first question.
My clutch hydraulics are all 40 yo, so I presume it's rubber.
I have left the MC et al, hanging below the car until I'm ready to replace, so I haven't pulled it apart yet. Visibly the MC grommet is rubber with the pipe inserted inside.
Pedal box is first and then I'll work on the MC and SC. Looking forward to cleaning up this area, as it is the last gooey, slimey, dirty, oily area left in the car.