Used during WWII, this was a letter addition square that was used to add OTP 5-letter groups to the plaintext groups in order to obtain 5-letter cipher groups. Frode Weierud expands on this.
"The table could have been made like a simple square with one vertical standard alphabet as the first left-hand column, however to minimize the risk of taking off the wrong ciphertext letter the standard alphabet has been copied in all of the columns. The substitution alphabets in each column are symmetric in the sense that if the entry for A is Ae then the entry for E is Ea. (see column B). The addition square is used to encipher by finding the corresponding OTP key letter in the top row such as to select the column to use and the cipher letter is the lower case letter in red standing next to the plaintext letter to be enciphered. Deciphering works the same way as the substitution alphabets are symmetrical as mentioned above. The silk square pictured below looks exactly the same as those issued by SOE to its radio operators".