I have seen a usage of would have v3 which do not fit into the known area of conditional type 3 and is used for completely different purposes.
In a court of law, a judge is asking a woman and a man questions about the case, where the woman accuses the man of stealing her money 2 years ago. So the judge asks them a lot of questions, to understand the case properly, like: How did it happen? When did it happen?
The judge asks the woman, +How old was he at the time he stole the money?"
The woman takes time to think and answers a few seconds later, "Eerrr, hmm.... He would have been 17."
So, she used "would have v3" for a guess or a speculation in the past, not for an unreal situation. It is interesting that she did not give the answer "He was 17". I have never seen such a usage. she could have said "He must have been 17" to empasize a strong guess in the past.
Why do you think she said "He would have been 17" but not "He was 17"? Can we use would have v3 for such a speculation or a guess in the past?
If yes, how will it differ from "must have v3" which is also used for a guess or an assumption in the past.