Actually, the clip bridge on the No. 1 Mk III it is held by four rivets, but the rivets are finished flat and the work is so well done it looks like the bridge is part of the receiver. It is part of the receiver on the No. 4 and 5 rifles, though.
The clip bridge is not a stressed component. Not only did the early rifles not have it, but as graywalker says, it is often removed when the rifle is sporterized.