I called a saddle shop that was selling a Mesace english saddle which had the center ring. She told me that she didn't have a girth for it either but had sold another saddle just like mine. It's an english style endurance saddle and uses aussie leathers. The gentleman she sold it to also questioned the girth, so he sewed on a strap with buckle which attached from the girth to the ring. She said it worked great. The saddle she sold only had the ring on one side. However she said that I could use the saddle with a standard girth without the extra piece.
I asked her about the overgirth and she said saddles with overgirths generally have a slit for the girth to slide through - so no, this saddle doesn't have an overgirth.
I'm going to give the saddle a try and see how it works without the extra strap. Thanks everyone for all of your help.