1950’s. The artist is elusive but we believe these great images were done by the well known Japanese artist Toshiko Okamura (also noted as Noriko Okamura) and the T.O. initials and the few examples of similar art we’ve seen lend credibility that she was the artist.
See http://artonmusic.blog96.fc2.com/blog-category-2.html for some exhibited art. The translation is a little flawed but you can scroll to her picture and then the images. Note the geometric backgrounds in addition of course to the music theme.
Great set of various year release promotional postcards along with “Record Times” envelopes from Nippon Gakki, better known as the Yamaha Music Co.. Really great condition with only a couple of edge marks. See pictures.