Movements

As you probably know already, there are two different movements used for the Speedmaster (Professional). The cal. 321 and the cal.861. Both are Lemania movements and are handwound mechanical. The current Speedmaster Professional uses the cal. 861 (or Lemania 1873 as you wish). The first generation of Speedmaster (Professionals) used the cal. 321 (Lemania 2310) till 1968.

"Omega launched the 27 CHRO C12 project jointly with Lemania in 1942, which led to the development of the new cal. 321 movement. The "27" in the project name referred to the movement diameter of 27mm. "CHRO" is a abbrevation for chronograph, while "C12" refers to a 12-hour timepiece. Omega was then using chronograph movements of three different diameters : 28.9mm, 33.3mm and 39mm. The company needed, however, a new 12-hour chronograph movement that was smaller and designed specifically for use in a wristwatch", source : Time Capsule page 142.

The big difference between the cal. 321 and the cal. 861 is the use of a "schaltrad" instead of a "shuttlecam". The cal. 861 is a bit more accurate (21600A/h instead of the 18.000A/h of the Cal. 321). The big decision of Omega to use the Cal.861 in 1968 was probably the reducing in cost. The use of parts was reduced and the expensive "schaltrad" chronograph system was dropped.

Below you'll see the two movements, the cal.321 at the top and the cal. 861 below. The current production of cal.861's uses a plastic part, which is not showed in the cal.861 movement below. The Speedmaster Professional which have the see-through case-back, use a Cal. 863. This is a cal.861 which was a bit restyled for better looks. The plastic part has disapeard in the cal. 863. Ofcourse ;-)

The power reserve of a Cal. 321 and Cal. 861 is about 50 hours (mine did 54 hours and 6 minutes).