The original 41P globe sight came with two inserts. They are about 1/2" in diameter and smaller than any one else made. You can spot one easily because they were the only ones that had three "tabs" that held them in place. They are very hard to find these days. You might check at this web sight as this gentleman was making a reproduction for around $20 a while back.
http://www.distantthunderbpcr.com/DTPages/DTProduct.htm