Rare 1964 IWC Ingenieur ref 666 AD, cal 8531 first generation ( second dated 1967 n after ).
21J. T SWISS T dial Antimagnetic to 80000A/m; 10 atm ; civilian version of Mark XI ! PAT.APPL Sn 1714650 dated 1963-64. Staybrite case sn 1823750 , all original n rare fish symbol! One of best IWC indeed !
https://www.vesperco.com/soldwatches/iwc-ingenieur-silver-dial-ref-666a
http://brittonswatches.com/watches_iwc_dates.htm
https://www.iwc.com/forum/en/discussion/68826/
http://oldwatch.club.tw/
http://www.moeb.ch/Ingenieur/01e_ref666.html