khmt2
·The date at 12 feature on the caliber 291 only existed from late every 1959 to very early 1961 on Conquests (going by serial numbers). Longines switched that feature to the caliber 341 in 1961 for the Flagship, ceasing to make that in very early 1962. I think, but have not collected enough evidence, that the 341 then got a date at 6 option on the Admiral and maybe the Grand Prize in subsequent years.
There's a 1968 date @ 6 341 Flagship for sale: http://www.ebay.it/itm/231451608332