When Montblanc saved Minerva they decided to invent new calibers/complications and they also decided to upgrade the level of finishing in existing Minerva calibers.
So when you compare pre-Montblanc calibers with them, you will see differences. Thus I can fully understand that a luxury company which upgrades existing levels of production wants to re-brand instead of keeping the old brand with its associated level of quality / finish.
There is a collectors booklet on the (now discontinued) product lines of Montblanc Villeret 1858 that succeeded the Minerva product lines in ca 2005. Please see pics attached.

