This is the Spiralwinder sapphire exhibition caseback for the Speedmaster Professional Mark II. Designed to reduce the thickness of the imposing Tonneau-style case by ~0.8mm, this caseback ensures that you have a complete edge-to-edge view of your movement. With it's modern stainless steel and optical-grade sapphire construction, you can watch the 861 calibre tick with confidence in your watch's durability.
 
How it's Made:
Our caseback consists of an outer ring, which we precision machine from 316L stainless steel. Then, we sandblast the slots for the opener tool, eliminating machining marks. 
 
After polishing the double-bevel to a mirror finish, the steel ring is carefully fitted with a scratch-resistant sapphire crystal. We apply a colorless, multi-layer AR coating to it, ensuring clear viewing and photos of your movement.