Joe, the rear sight is a Lyman 48 aperture sight. The threaded hole on top of the sight will accept different sized apertures, or a variable aperture like you apparently have.
The Lyman 48 is a great sight and was used on a number of 03 match rifles.
How does yours shoot? Can you post any pics of yours? Do you have any information about who built yours?
I took the hand guard off mine and discovered that someone had gone to a great deal of trouble to see that the barrel was free floated in the stock. The only contact is a slight downward pressure on the barrel by the top barrel ring.
