master cylinder rebuild

[dunk 1]

have decide to do a complete rebuild on my brake seals. Callipers are easy but I have a few questions about the master cylinder - imagine the piston laid out in front of me - - spring end to the left lever end to the right. There are 3 grooves - the one furthest to the right gets the simple 0 ring like seal. there are then 2 more grooves - a flat sided and based one closest to the spring end which also has a hole in it and one with rounded base and sides and no hole between the other grooves but closer to the spring end. My question is which of these grooves does the other seal go in. It has a groove and wider end on it which I presume faces toward the spring. Now it came off the middle curved groove but diagrams on the web - but I cant find model specific one -seem to suggest this seal goes in the one nearest the spring - a bit worrying if I have been riding all this time with it in the wrong one. also why is the spring end of the piston internally threaded - theres nothing to go there ? thanks - Duncan