The letters carry printed hotel names beside the handwriting. Their envelopes have stamps and postmarks.
Three hotel letterheads
The Tokyo letter is on Hotel New Otani stationery, the Kyoto letter on Kyoto Hotel and the Osaka letter on Hotel Osaka Grand. The same names appear on the envelopes.
The Kyoto letter is written in green ink, the Tokyo and Osaka letters in dark ink.
Why both sides matter
The Osaka letter shows why the back matters. It opens with 20 July, without a year. At the bottom right of the back it says Osaka, 20 July 1967, Japan.