jeff,could well be that the bolt was altered for the second bases that it was tapped for ,does the gun have any british proofs ,they have always produced some out of the ordinary scope bases judging from how old the gun is i would think anyone who wanted a scope on there rifle would order a square bridge when it was available, and not go thru the problem of rewelding bridges and machining them ,that is why i think that they must be silver soldered ,although the rear bridge does look like continuous metal,for now if you want to call it a "prototype square bridge" it is ok with me!! ...paul