Omega SMP , Caliber 2500 or caliber 2628?

I found the following info in another forum...

"2628B is the most current version. The reason things are so confusing is because Omega started out with caliber 2500A, which was the fist regaular co-axial movement with just the date function. This movement was problematic, so none of the other movements like 2628 were based on it. When the newer and properly functioning 2500B came out, Omega used this as the basis for 2628A, the first GMT co-axial movement.

About a year and a half ago, Omega came out with 2500C, which went into all of the Planet Ocean models. This version has a slower beat rate and a modified lift angle to make production easier. Omega then used 2500C as the basis for 2628B, which is the current version used in the co-axial GMT watches.

ETA 2892-A2 is indeed the base movement, but by the time Omega makes all of their modifications, adds the co-axial escapement, the larger, free-sprung balance, and all the other upgrades, the resulting movement looks nothing like its base ETA.

If you are looking for specifics, the older 2628A had a beat rate of 8 beats per second (28,800 per hour). The newer 2628B beats at 7 beats per second (25,200 beats per hour). This results in about 3.5 extra hours of power reserve, since the 2628B isn't beating as fast. Also, less work theoretically means less wear and tear on the movement, which is one of the reasons Omega made the change. George Daniels, who created the co-axial, recommended these modifications."

The GMT comes with a displayback as well if I remember correctly