The Montblanc caliber 4810-502 is based on the ETA/Valjoux caliber 7753. This caliber number was recently updated to MB 25.02.
Some of the topics covered in this caliber listing:
Not a 7750:
You may see many bloggers or watch sellers incorrectly listing the base as an ETA 7750. The difference, which makes for the signature styling of the Montblanc Timewalker featuring this movement, is the placement of the subdials. The 7753 has the subdials at 3, 6, and 9. Another differentiator is that there is date pusher on the side of the case at 10:00. This is why the Montblanc Timewalker includes a special pusher tool. If the movement were based on a 7750, it would be possible to set the date via the crown.
Montblanc 4810 502 vs ETA 7753:
Montblanc made a few changes to their caliber 4810 502 compared to the standard ETA 7753. In most timepieces with the 7753 you will find the date at 6:00, however, Montblanc fit the date between 4:00 and 5:00.
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.