These are the Mondaine stop to go swiss railway watch straps. They are 20mm width leather straps with stitching in two colours, black and red. There is also a silicon version. Fitting the strap is easy, and this page will hopefully give you the method and some tips. The silicon strap requires some special attention to shorten to the required length, again, this link should help. If you are confused about which watch is the “Stoptogo”, this video should help.