I thought the 'frog' was there to make it easier to make hand moulded bricks. A wedge shape set in the base of the mould forced the clay into the corners when the right size lump of clay was just thrown in, without the brick maker (who was probably on piece rate) having to mess about tamping the clay into the corners of the mould to make a square brick.
It probably makes the bricks quicker to dry after moulding and easier to fire evenly as well.
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