The Heuer Bundeswehr Ref. 1550SG is a legendary, very rare and highly collectible Cold War-era military aviation watch. Originally manufactured by Heuer-Leonidas for West German military pilots for NATO, (the Bundeswehr) during the 1960s and 1970s, it is renowned for its bold presence and exceptional utility.

  The code 6645-12-146-3774 is a military supply number (NATO Stock Number) engraved on the caseback of the iconic Heuer Bundeswehr 

  The breakdown of the NATO Stock Number (NSN) signifies its military origin:

• 6645: The NATO supply classification code for "Time Measuring Instruments."

• 12: The country code designating West Germany.

• 146-3774: The unique item identification code for this specific Heuer model.

• Case Size: 43mm diameter (brushed stainless steel with a two-piece front-loading construction). 

• Movement: Valjoux Caliber 230 manual-winding column-wheel movement. 

• Flyback Function: Allows the pilot to instantly stop, reset, and restart the chronograph hand with a single press of the lower pusher—crucial for precise, consecutive navigation timings. 

• Dial Layout: Matte black dial featuring stark white Arabic numerals and dual sub-registers (running seconds at 9 o'clock and a 30-minute counter at 3 o'clock). 

• Bezel: Thick, bi-directional rotating black aluminum bezel for tracking elapsed hours/minutes.