Helvetica MH1.L2211.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.L2211.LB watch strap, 18mm Black Leather IP gold plated buckle.
Helvetica No. 1 Light 38mm diameter.
Some watches come in different sizes, so please check the width of your strap and/or the diameter of your watch.