This Speedmaster known as the 'CB Case' is rare because nearly all Cal. 321 Speedmaster cases were manufactured by the Omega case-maker, Huguenin Freres & Co, in Le Locle, Switzerland and thus signed "HF" inside the case-back. Only in 1966 with the reference 105.012 did Omega use an additional case-maker, and signed "CB" in the case-back. These cases were only manufactured during a very short period of time and are extremely rare. This case has a particularity known as 'flat lugs'. The lyre lugs on these cases have flat edges at the top, making them easily recognizable compared to other lyre lug cases.