Fune no kagaku kan (Museum of Maritime Science) and MES Tokki Co., Ltd. have furnished the drawing.
MES Tokki is a subsidiary company of MES(Mitsui Engineering and Shipbuilding Co., Ltd.).
[ Part of drawing displayed at Museum Mikasa. ]
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As you know, JACAR has put several other drawings of Mikasa on view via web.
[ Plan of main deck of Mikasa (subjoined pages by me)]
( JACAR http://www.jacar.go.jp/index.html ; Ref# : C04015499200 )
Ref# C04015499100 contains plan of middle deck of Mikasa.
This drawing was made about 1926 for preservation of Mikasa, but perhaps it was copied from drawing at completion of Mikasa.
Original drawings (main deck & middle deck) can be viewed at NIDS "The National Institute for Defense Studies".