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 264 is a mid-sized piston filler. My pen dates back to 1954-56. The nib on this one has medium flex and allows for some expressive writing. In my opinion, it is a rather classy design. The 264 looks very much like the 254 but for a few critical differences. First, it has a very different nib - a rather typical looking MB nib. The 26x series was cheaper than the 25x series. But, one of the catalogues lists the 264 as a "special model" and it is advertised as having a "streamlined design".
The 264 cap and body resembles the 25x and 34x series of the mid-1950s that were now made using injection moulded plastics. The 264 is a screw cap unlike the 25x. The pen with the blue ink window in the picture is the 264. Notice how closely it resembles the 256.
Rösler, J., & Wallrafen, S. (2001). Collectible Stars: Montblanc-Schreibgeräte von 1946 bis 1979.