Had the chance to look closely at a WR250R this weekend (thanks, Bonnie Abbzug!) and want to clarify what to order for that bike.
The stock mirror includes an R/H - L/H adapter which is under the rubber at the lower end of the stock right mirror. You can wind it off the mirror, and use a RAM-B-349 (Ball stud w/ 10mm x 1.25 normal thread) that you get from us or anyone else.
Therefore, I recommend ordering the same parts for both left and right side, and using the stock adapter.
Here's a picture of the adapter with a Ball Stud threaded in:
I'll update my web page shortly, figured I'd start here. :-)
Thanks, everyone, for the great reviews!