No. 342 D
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.
The 342D is the pen that threw me down the vintage rabbit hole! It is the smallest pen in the lowest series of the time - very humble. It was issued in several colours like burgundy, petrol blue, grey, green, and black. Mine is black and has the blue cap crown which signifies that it was made for use with carbon paper. This pen was probably made between 1957-58. Old catalogues show that the 3xx series came with only a 10-year warranty, whereas the 2xx series came with a 25-year guarantee and the 1xx came with a lifetime guarantee. 342 was suggested as the right size for women and youth. Also, it came in three versions - the gold plated version which was the cheapest, the regular (which I assume was gold as it was 50% more expensive than the gold plated), and the 'D' nib which was even more expensive than the gold nib at 17.5 DM vs the 15 DM gold nib.
The 'D' in its model number and on its nib signify the same thing i.e. the nib is a hard nail for the purpose of using pressure to write through many layers of carbon paper. Rigid nibs are often looked down upon with all the attention being given to flexible nibs. However, my experience with this D nib and also rigid nibs on my earlier vintage Sheaffer pens is that they hold really well in terms of alignment and condition over time. Once you tune them, they don't de-tune with use.
Rösler, J., & Wallrafen, S. (2001). Collectible Stars: Montblanc-Schreibgeräte von 1946 bis 1979.