This beautiful emblem has been incorrectly entitled "Johrei®" (or "White Light") since it was published in Kathleen Milner's 1994 book Reiki and other Rays of Touch Healing. That inaccuracy was repeated a year later, in Diane Stein's Essential Reiki.
According to James Deacon, the actual translation of the 5 kanji characters Dai Ko Myo Shin Shin is "Great Bright Light of the True God" or "Great Enlightenment of the True God." Ray Toba of the Johrei® Fellowship interprets it as "Great God of Light."
Mrs. Takata made Iris Ishikuro a Reiki Master in 1979. Interestingly, she was the first Takata-trained Master to stop charging the traditional $10,000 fee for Reiki Master training.
Ishikuro later moved to San Jose, and became a member of the Johrei® Fellowship. She obtained a copy of the Dai Ko Myo Shin Shin (probably on a scroll) - which was considered sacred by Fellowhip members.
I believe that, when she noticed the Dai Ko Myo portion of the emblem, Ishikuro assumed it had metaphysical correspondance to the Usui Master Symbol - which is made up of 3 of the 5 characters in the emblem - and felt it would be a great energetic adjunct to her Reiki practice & teachings.
It is known that Ishikuro hand-made copies of the Dai Ko Myo Shin Shin, and used these as a kind of "power symbol" in her Reiki teachings.
When I received my training in Johrei* Reiki, there were several copies of this emblem posted throughout the classroom.