from Armin at posteroid.com
Horizontal Three Sheets
Three sheets were issued until the late 60’s and are now very rare. They consist of three separate B1 sheets that together form a horizontal design.
Yes, I've seen numerous horizontal billboards (3xB1), but this is the first time I'm seeing a vertical billboard.
Doesn't surprise me though... while Japanese posters are almost always printed on standard paper sizes (B3, B2, B1, B0), on occasion they played around with the layout of multi-sheet posters. For instance, the vast majority of B0's from around this time were printed as 2xB1 (29x40 each) and arranged side-by-side to result in either a 40"x58" portrait poster or a 58"x40" landscape poster. However, I have a B0 that is still made from 2xB1, but they turned the B1s sideways so the poster is 29"x80" panoramic landscape!
I also tracked down vertical three sheet that is arranged just like your poster, but it made from (3) B0 posters -- so it's nearly 5' wide and 10' tall!