Hi Goldenlutin, Oliver...
The MB 29.15, as Robin has replied, is the name given to the movement which is the Sellita base + Perpetual calendar module TOGETHER .
And so the exclusivity to Montblanc is for this combination...not the SW 300 itself.
Surely this is nothing new in the watch world correct? A company using a common base tractor with a custom complication module and renaming it
Cheers, Mike