Saw vise question

[Charlie Buchanan]

I recently bought a Wentworth's No. 1 saw vise to replace a wooden shop-made vise I've been using. It works quite well but I'm wondering what the dovetail slot in the movable jaw is for. It runs the length of the jaw below the gripping surface--see photos. Anyone know what the slot is for? The vise seems to grip very well as it is with nothing in the slot. Just wondering why it is there. Thanks
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[ray hampton]

I am not the maker of this vise so I could be wrong but the slot may be for a dovetail jaw insert of a type that will stay in place without pinching your finger while you hold it

[Maurice Metzger]

I seem to recall seeing an old ad for this vise that showed a rubber insert in the slot to cut down on screeching while filing.