Hi, just found your post! The 403 is a fabulous trailer. We had a 505, but nearly always just towed one horse in it, when we decided to upgrade to the newer style we went for the 403. It is fantastic to tow, much lighter. And the 403's are known as 'mare and foal' boxes, so they have far more room in them for the horse than they normally have when partitioned in a 506/511. Much more room for the horse to find his feet.
I can highly recommend them!!!