In the 1970s, the Swiss watches suffered in the crisis of quartz watches and Omega began to simplified the structure of calibre 321. So the calibre 861 was born, which is now the calibre 1861. It was usually equipped in the watches with the solid caseback. So it is usually not with the polished finish. See the picture please, the Omega Speedmaster copy with black dial has been just equipped with the calibre 1861.
The calibre 1863 is exactly the calibre 1861 which is with the polished finish and it is often equipped into the timepieces with the transparent caseback.
Omega has a profound and deep relationship with the moon, so the moon phase function is exactly in line with the moon watch. So the calibre 1866 is exactly the calibre 1863 being added with the function of moon phase. With the moon phase and date display, the price of the 44.25 mm Omega imitation watch is much more expensive.