The german "Luftwaffe" Leather Bund Strap
The main advantage of these straps is that no matter how hot or cold the watch gets the leather behind the watch keeps it away from contact with the wrist. Something I hadn't realised until someone pointed it out to me quite recently was that if you remove the backing it can be turned into a conventional two-piece strap. Once you've done this it's pretty logical how straightforward it is but it's strange how you overlook the obvious. You can see the component parts above.
This strap is a classic and unique European military watch strap which looks excellent on almost any military or civilian watch with 18mm or 20mm lug size, the overall length including the buckle is 21.5cm, the part where the watch sits is 50mm. There are a number of manufacturers but the one which I found that seems to be a realistic price is the MWC version which looks virtually identical to the straps made in the 1970s. They charge $20.00 / £14.95 / €16.50 which seems quite reasonable compared to many of the companies I've seen where they are very overpriced and also some quite interesting images of the strap at the link below.
These three companies are alternative suppliers of these straps:
https://www.vintagestraps.it/product/vs-hand-made-strap-black-leather-with-bund/