I have a number of Langes in my collection and while the quality and design of their OEM straps is excellent I like the option of having different strap colors to swap out from time to time. I also spend a lot of time in a hot humid climate so the option of having straps with caoutchouc linings is preferable for the summer months.
While I have purchased straps for Jean Rousseau and ABP Concept for other watches in my collection, I have found that the straps from Camille Fournet are superlative in their fit and finish. I have included photos of their Taurillon Calf (Tobacco) with caoutchouc lining that I purchased for my Datograph. From personal experience their web configurer offers more options than others and as you can see the quality of the finishing is first class (hand stitched option on this). It’s nice to be able to tweak the strap lengths from the 115x75 standard that ships with their watches. I use 110x75 for my Lange 1, Richard Lange, Annual Calendar and this Datograph, while going with 110x70 for my Zeitwerk Striking Time, where the lug to lug measurement is measurably longer.
BTW - Lange don’t seem to include the buckle sizes anywhere on their website so here are the sizes for the watches mentioned. Saxonia Annual Calendar, Lange 1 (38.5mm) and Richard Lange Jumping Seconds are all 20/16mm. The Datograph is 20/18mm and the Striking Time is 21/18mm. 


