77777...77777 (49-digits)

This number is a prime.

                       777777773 3333773777 3773737377 3777377333 3377777777

      7 7 7 7 7 7 7
      7 3 3 3 3 3 7
      7 3 7 7 7 3 7
      7 3 7 3 7 3 7
      7 3 7 7 7 3 7
      7 3 3 3 3 3 7
      7 7 7 7 7 7 7

The only square-congruent prime of order 7 that contains two prime digits. [Hartley]