Helvetica MH1.R2220.LB watch strap.
It is possible to fit this strap yourself, it is a little fiddly but relatively easy. Check out this video.
Having trouble finding your strap? More help is here
Helvetica MH1.R2220.LB watch strap, 20mm black stitched leather.
Helvetica No1 Regular 40mm diameter.
Some watches come in different sizes, so please check the width of your strap and/or the diameter of your watch.