Mark is correct about the metal tacks eating up the bass strings. So will the "paper fastener" type that hang on little leather or bellows cloth tabs, if they installed in a rail by the bass strings. All "mandolin rails" I've seen just engage the tenor and on up. A better solution is to use hard fibre squares or half-round wood dowels glued to the tab ends. There are other myriad ways of achieving the effect, including making a separate rail that carries little plungers of wood with "paddles" at the ends that are spring loaded. When a choke-type cable is pulled, the rail brings the little paddles in line with the hammer shanks. The shanks hit the paddles and the ends hit the strings. When the choke cable is pushed in, the line of the little wooden dowels moves in between the shanks so as not to be hit by them. Berley
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