Esterbrook M2 Aerometric Pens
Esterbrook jumped on the bandwagon in the late 1950's with an aerometric filling, metal capped fountain pen called the M2. These came in numerous colors in a number of different variations. The plastic used is a bit softer than earlier pens, but typical of late Esterbrook manufacture. Don't be fooled, however, these are pretty durable pens.
M2's are easily recognizable by their indented cap and plastic threads. Here's an example of red and olive colors. M2's can be found in two main types, with the imprint on the cap band as shown in the red example, and on the clip as in the olive example.
Here's an example of one Esterbrook's funkier colors, turquoise, with the matching twist pencil.
Some more M2 Pens. The Black example is not perfect, and displays the softer plastic by way of the dental impressions on the end of the barrel. The Blue example also has its own issues, with the inner cap just peaking out beyond the cap lip. This is fairly common as these caps can split rather easily. Pencils are twist style as most later Esterbrook pencils are, and ballpoints are push activated. The last ballpoint in grey is unusual in the fact that it has the Esterbrok imprint on both the clip and the cap band.