Let $\backslash$delta represent some fixed letter of the alphabet. Della encrypts the plaintext PAWNSHOP with the message key AB$\backslash$delta $.$ The ciphertext includes some repeated letters in the form $\backslash$alpha $\backslash$beta $\backslash$beta $\backslash$alpha $($that is$,$ the $1$st letter is the same as the $8$th letter, and the $4$th letter is as same as the $5$th letter$).$ Determine with justification all possible values of $\backslash$delta or justify why none exist.

Avoid an $$exhaustive attack approach: do not encrypt the full plaintext $26$ times.