Since the late 1960s, Pelikan had been producing piston filler pen M series (e.g. M30, M60). These were a competitor to Montblanc's two-digit fountain pen series (e.g. No.14, No.22) (Pelikan Pens: History). However, while Montblanc kept its classic flagship No.149 in its lineup, Pelikan did not have a classic style flagship model in its true sense after 400NN was discontinued in 1965. This situation remained the same in the 1970s.

In Japan, the classic style piston filler pen model 400NN, which had long since been discontinued, continued to enjoy great popularity. Under these circumstances, in response to strong demand from Japanese retailers, Pelikan decided to reissue 400NN (and 120) for the Japanese market. However, after stopped producing models 400NN and 120 in 1965, Pelikan had abandoned its manufacturing facilities and concentrated on "the Pelikano business". So, Pelikan did not have the necessary manufacturing facilities and had to outsource their production to another company, Merz & Krell (Pelikan: Ein Unternehmen schreibt Geschichte)⁽¹⁾. 400NN was finally produced by Merz & Krell in 1973, eight years after 400NN was discontinued. There were four types of barrels: green striped, brown striped, plain black (with green ink window), and black striped (万年筆おやじの備忘録).

Merz & Krell version had many subtle to notable differences from the original 400NN (万年筆おやじの備忘録,  万年筆おやじの備忘録, 万年筆おやじの備忘録, 万年筆おやじの備忘録, 万年筆おやじの備忘録, Pelikan’s Perch).

• The cap top had a different diameter (M & K: 7.25 mm vs. original: 7.35 mm).

• The crown had a different construction (M & K: solid metal vs. original:plastic and metal) (Fig.4).

• The clip had different shape (slightly wider and longer) and different engravings (M & K: Rg 400NN vs. original: VR) (Fig.1-2, 5) .

• The collar had a different construction and a thread (incompatible with the section of original 400NN, and vice versa) (Fig.1-3).

• The feed had different construction and diameter (M & K: 5 mm vs. original: 5.5 mm)(incompatible with the collar of original 400NN, and vice versa) (Fig.2-2, 2-3).

• The nib had a different Pelikan logo (M & K: one chick vs. original: two chicks) (Fig.2-1).

• The nib had no positioning hole at its base (Fig.2-1).

• The section had a different thread (incompatible with the cap tube of original 400NN, and vice versa) (Fig.1-3).

• The section had a different edge shape (Fig.1-1).

• ​The barrel was shorter, while the turning knob was longer. The overall length was 2 mm longer for M & K (Fig.1-4).

• The turning knob was larger in diameter (as a result, there was no step between the turning knob and the barrel)(Fig.1-2).

• The piston mechanism was screw-in fitted rather to the barrel (original: friction fitted) (Fig.7).

• The piston seal was integrated with the piston rod (!) (Fig.6).

 

Given these differences, it is clear that Merz & Krell did not simply make a copy of 400NN, but went to great lengths to produce the best fountain pen possible at that time.

Fortunately, 400NN (Merz & Krell) hit the jackpot in the Japanese market. At that time, not only Pelikan but also the fountain pen industry was in a dark age (Ravens March Fountain Pens, Pelikan: Ein Unternehmen schreibt Geschichte), and although I do not know how much the 400NN (Merz & Krell) contributed to Pelikan's performance⁽²⁾, there would be no doubt that it led to the reissue of 400 as Souverän M400 in 1982⁽³⁾ and the subsequent revival of Pelikan in the 1980s.

Note:⁽¹⁾The official name was Merz & Krell GmbH & Co. KGaA. The company (was founded in 1920 and began manufacturing writing instruments. Except for having to stop production during World War II, the company has continued to grow and prosper. It is best known for its Melvi, Senator, and Diplomat lines of pens. In January 2006, the company changed its name to Senator GmbH & Co. KGaA and continues to manufacture pens today (Pelikan’s Perch).
⁽²⁾In 1978, Pelikan became to a stock corporation, with all shares owned by the Beindorf family, and the company was reorganized. However, the company's 40 subsidiaries, which published books, manufactured toys and cosmetics, and even produced animal feed, continued to operate at a loss, and under the leadership of new president Kurt Hackel, who took office in 1982, unprofitable businesses were liquidated (Pelikan: Ein Unternehmen schreibt Geschichte). 
⁽³⁾In Japan, 400NN (Merz & Krell) was simply sold as "400". 400NN (Merz & Krell) was sold in Japan at least until 1980 (万年筆評価の部屋). When Souverän 400 was introduced in 1982, it was called "500" in Japan for a while to avoid confusion with the Merz & Krell version of 400 (万年筆評価の部屋).