20mm Hirsch Duke on mine (old photo). You can see a tiny gap between the strap and lugs if you look closely. Hirsch produces 21mm straps. I guess 21mm straps would also fit on 20mm lug width watches. The reason I bought a 20mm strap instead was so that I could use it with multiple watches, but to be honest I haven't used this strap with any of my other watches.