I have a Shimano (I think Dura Ace) 10-speed rear shifter from around 2008, it came on the aero bars of a Cervelo P2-SL. Like a genius I took it apart today and I cannot for the life of me work out how it goes back together. Not much on the web though many people refer to a '105' bar end shifter though mine says '10S' (10 speed), it looks a lot like '105' so I think that is the cause of the confusion.
Here are the constituent parts:
And with the four rightmost parts turned around:
I haven't found the exact part on Shimano tech docs. The closest I find is the SL-BS78, but the exploded view PDF makes no mention of the two irregularly-shaped washers in the centre of my pictures, and they're my problem. I did find what seems to be the same mechanism in the SL-7900 downtube shifter, relevant bit shown here:
It's the two washers that are killing me. The rightmost one (part 5 'Stopper washer' in the EV) fits in the lever nicely, but the leftmost one (part 6, 'Spacer for rear'), while it does sit over the mounting stud, freely rotates when it seems it should engage on something. It has 6 tabs that correspond to the mount, and seem to correspond to the 6 lugs on the SIS unit (lower photo, right), but it doesn't fit over or inside any part during assembly. It slides around and it seems it needs to line up with the SIS unit somehow.
My washer 6 has a tab (lower photo) that goes toward the mount. The 7900 EV doesn't show that, but the Service Instructions show its orientation, which I'm following. It's just that I can't get it to line up with or over the SIS unit.
Help me, wise interwebs.
EDIT: November 2020
Short answer: I kept fiddling with it, and eventually it all snapped together nicely. So there is no magic orientation, you'll know when everything is correctly assembled. The answers below are much more methodical, and helpful.