In 1965, all previous piston filler models (400NN, 140, and 120) were discontinued, and M series (piston filler pens with the letter M at the beginning of the name) were introduced (e.g. M20, M30, M60). In 1966, the luxury model M100 (all in solid gold) was introduced. In 1967, M10 was introduced.
 
Between 1966 and 1967 (1968?)⁽¹⁾, less expensive MK⁽²⁾ series (piston filler pens with plastic cap and steel nib) were introduced (MK10, MK20, MK21, MK30)(Pelikan-collectibles.com). 

Different from other pens, MK10 had a stylized beak clip (Fig.3)⁽³⁾. MK10 with black and green barrel (Fig.1, 2) was also sold as "120 black-green" (Pelikan-collectibles.com)⁽⁴⁾. The cap top was integrated with the clip (Fig.4). This pen also has two features⁽⁵⁾.

• The piston seal, in addition to the conventional white seal, had a green seal added to the back and was held in place by a metal ring (Fig.5). This is thought to be a measure to prevent ink from leaking backwards.

• "The turning knob indicator" was added to the filling mechanism (Fig. 6). Turning it clockwise sucks in ink. Turning the knob counterclockwise again ejects ink, but after a little more than one turn, you will feel resistance. This is an indication of how far the knob should be turned to discard excess ink in the section and thermic-regulator. Finally, tighten the turning knob with the nib facing up. If these steps are not done, excess ink in the section may leak out during writing. The instruction manual recommends that you squeeze out 7 to 8 drops of ink after sucking (Fig. 7)⁽⁶⁾.

In line with the revision of Pelikano in 1968, M/MK series was revised between 1969 and 1970 (Pelikan-collectibles.com).

Note:⁽¹⁾According to Pelikan Schreibgeräte, MK30 (also known as M25) was introduced in 1966. MK10 was introduced in 1968.
⁽²⁾"K" denoted "Kunststoff"(plastic).
⁽³⁾Only M10 and MK10 had a stylized pelican beak clip (Pelikan-collectibles.com).
⁽⁴⁾The green barrel and black cap were reminiscent of the previous model 120.
⁽⁵⁾Presumably, these updates were made not only to MK series, but to the entire late piston filler pen.

⁽⁶⁾In the Souverän series that followed the M series, the feed was exposed so that excess ink could be wiped off with paper or cloth, eliminating the need for these steps.