Adrian it's been debated on here before and I don't think anyone has ever nailed down it's evolution. The earliest writing about it that I can find comes in the 70's and yet the oldest members say it certainly hasn't changed since they were boys.
This section of the course includes the Redan green and the Pit which have been there practically since the beginning. However in some early maps the green looks square and surrounded by ditches (the wall in the starters office has one). Perhaps one was covered and became the swale? But no one has ever found a report saying it was altered.