1.2 millions of records from NDL Japanese Periodicals Index's "Science and Technology" and "Natural Science" published from 1950 to 1974 which has been not incorporated, is now available in CiNii. Japanese Periodicals Index is one of the databases incorporated. It's made by National Diet Library. The code at the CiNii window is "NDL".
This will enable you to search the same range as NDL-OPAC (by National Diet Library) via CiNii.
You can see the list of databases incorporated in CiNii at the following page.