Seiko Instruments Inc. VD51B

The SII caliber VD51B is a quartz chronograph movement manufactured by Seiko Instruments Inc. in Japan. It operates at a frequency of 371 Hz and maintains accuracy within ±20 seconds per month across a temperature range of -5°C to 50°C. The movement features a small seconds subdial at 6:00 and uses an SR920SW battery with an approximate life of three years. The VD51B remains in production and is used in watches such as the AVI-8 Bulman.