The resevoiur, the residual valve and probably a small difference in the springs.
http://www.imperialclub.org/Repair/Lit/Master/232/Page04.htmBe cause they aftermarket replacement companies are sell what is
good enough' and probably 'close enough' its unlikely anyone will take them to court when they wreck.
To convert a drum master to the disk master all they do is remove the residual valve for the disks (rear port).
You can do the same - see page 9.
Filmstrip if you prefere A-V
http://www.imperialclub.org/Repair/Lit/Films/230/Might want t olook throughthis one as well
http://www.imperialclub.org/Repair/Lit/Master/233/page12.htm
