I was reading this thread as im planning on fitting a new Brake Master Cylinder(15/16") to my BB4.
I have a brand new OEM MC and an old MC and servo/booster so i thought it would be a good idea to have a look at them:
Old mc out of Merlins BB4. Pretty rusty

Note no seal on MC

The only seal is lodged in the servo/booster

New Honda Brake MC - no seal supplied with this.

The seal has part number on the back 10011-142-1100. On the front it says M/P Side.

Old seal fitted to MC
Hope this adds to the knowledge
