Translation:Between her cold fingers, they found a letter.
The letter (the word dopis) is at the end of the sentence, it is the new information that the sentence presents. Therefore "the letter" would really require "ten dopis". If it were "Dopis našli..." it would be different, because it is clear we should know about the letter already.
Check the answer to LaynePierc1
The sentence really tells that there was a letter, it is the new information given by the sentence. "The letter" (a letter we already know about) would have a different word order and very likely also a demonstrative "ten dopis". Like: "(Ten) dopis našli mezi..." or "Našli ten dopis mezi...".