All MWC 500M diver models come with a helium escape valve
which appears on their website at this link
Enhanced Nighttime Visibility: Utilizes Super Luminova for improved nighttime visibility while retaining the historical encircled T on the dial
40 mm incl crown
but there's now a simpler alternative
20mm FKM (Patterned) Rubber Strap with Quick Release Feature for Fast and Easy Fitting and Removal 18mm NATO Straps All MWC 500M diver modelsFKM2 FKM, short for Fluoroelastomer, is a high performance synthetic rubber known for its exceptional density, durability, and resistance to water, chemicals, oil, and heat. Originally developed for aerospace applications, FKM was designed to meet the stringent reliability and longevity requirements of aircraft and space missions. These qualities make FKM straps highly favored by military personnel, police officers, and professional divers, as they