The Montblanc caliber 4810-503 is a Swiss made, mechanical, self-winding automatic GMT chronograph movement based on the ETA/Valjoux caliber 7754. This caliber number was recently updated to MB 25.03.
Not a 7750:
You may see many bloggers or watch sellers incorrectly listing the MB 4810-503 has being based on the ETA caliber 7750. This movement is actually based on the 7754. The main difference is that the 7754 features an additional GMT complication – in other words, there is a central GMT hand for tracking a second time zone. Another difference between the two is that the 7750 features a day/date at 3:00, whereas the 7754 just has a date at 3:00.
Caliber number confusion:
Montblanc is yet another example of a watch brand not putting too much thought into their caliber designations. It is easy to get confused by the 4810-502 and 4810-503 calibers because the 502 is based on the ETA 7753 and the 503 is based on the 7754. It would have made more sense for MB to match them to the last digits of their base calibers.