Mine measure 16.2mm internal diameter, no flair. (Stock bars measurement)
I use these on the new bars but they should fit the stock ones fine too:
https://smile.amazon.com/gp/product/B01AVHQB22/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1
Cheers!
Hafny Bar End Bike Mirror, Stainless Steel Lens, Safe Rearview Mirror https://www.amazon.com/dp/B01AVHQB22/ref=cm_sw_r_cp_api_i_xTcrFbJTK9MEM
You can get the other poster's mirror on Amazon too.
Anyhow. You may not like the small'ish circles that plug into the end of the handlebars. But they work great and you really only need one on the left handlebar (right if you're British). I have 3 bikes now (2 Lectric and 1 Niu).
My Lectrics use a cheap plastic version, but got this one for the Niu-Bqi
https://www.amazon.com/gp/aw/d/B01AVHQB22/ref=ox_sc_act_image_2?smid=A2ON4K5IJDY2SV&psc=1