The NEC M-System was a Japanese input method and a series of word processor keyboards introduced in 1983. Posted by wj-zhe.
KBD.news Published November 11, 2022
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According to an article by the IPJS Computer Museum, NEC announced M system, a new input system for Japanese language word processors, in March 1983. The "M" comes from the name of the developer of this method: NEC special consultant Morita Masasuke.
At the time, input speed was an issue for Japanese language word processors, due to the mixture of kanji (Chinese characters) and kana (phonetic letters) in sentences. Various methods were developed to improve input speed, including: the kana-kanji conversion system, the romaji (alphabetic letter) kanji conversion system, the pen-touch system, the so-called "kanji-teletype" system, and the association system where kanji were input with 2 kana characters. Each of these methods had advantages and disadvantages. (source)
The author of this new method noticed that the "on-yomi" pronunciations of kanji could be divided into 5 patterns, and came up with his system where text input was done using 3-row, 5 column romaji keyboards.
This system made it possible to input text at a speed more than two times faster than the kana-kanji conversion system.
The first keyboard based on this input system made its debut in August of 1983, accompanying the PC-8800 Series word processor, marketed as PCWORD-M.
PWP-100 (source: IPSJ)
In 1984 NEC entered the personal word processor market with the PWP-100 personal Japanese language word processor, followed by a number of M System word processors.
I have another version of an M-type keyboard in the split database: