In your second picture it is not clear if there is some kind of clasp on the back strap or if it is just an attached piece of stretchy fabric. It's one size fits all but from the pictures I can't tell it the back strap stretches or adjust or both.
Does it look something like this one on the back? http://www.amazon.com/Ben-Sherman-Mens-Baseball-Cap/dp/B00TDFCWHW
Some other Ben Sherman hats http://www.lids.com/Ben-Sherman/Hats
Have you had this hat a long time? In my experience with these kinds of hats they make style changes especially designer hats that make it difficult to find the same hat after some time.