All the photographs in this book, unless stated otherwise, date from the 1950s.
Can I replace "unless stated otherwise" with "unless they are stated otherwise"? Does the sentence make sense to you?
English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English. It only takes a minute to sign up.Sign up to join this community
According to Miriam-Webster, the meaning of "to state" in this context is
"to give (specific information, instructions, rules, etc.) in writing"
Hence, "unless stated otherwise" refers to the possibility that the writer may or may not state that the date of a certain photograph is different. (http://www.merriam-webster.com/dictionary/state)
Given the above meaning, I do not see how the statement "the photographs are being stated" makes sense or is equivalent to the short form "unless stated otherwise". A suitable longer version is (see also comment by Damkerng T.)
unless it is stated otherwise