The Hamburg factory recovered strong after being bombed in the mid-1940s. It introduced a significant technological advancement in the 1950s - injection moulding. This allowed for faster, cheaper, less labour intensive, and more durable and reliable pen manufacturing. In fact, this technology changed manufacture efficiencies so much that the Danish and Spanish factories could no longer compete with the German one and so they seized operations eventually. The 34x, 25x, and 26x are the first few series produced using this technology.
To me, the 25x series has special significance in MB's history because of its unique nib design.
For bibliography, see Resources page -->link