I want to replace the mirrors but don't what size I am supposed to

I want to replace the mirrors but don’t what size I am supposed to get any help I this?

20mm is standard.
There is SI and MO (and some others) to choose from, SI is a bit better, MO is tougher against smoke and cleaning while SI is very delicate.
Keep away from K9 mirrors - far away!

#K40optics