The punches (on the top and) bottom of the bar symbolize the sky (rosette in a circle) and earth (3 lines in the square). As with the Chinese cash coins, where the circular coin shape is the symbol of the sky and the quadrangular center hole is that of the earth, the ruler, as a mediator between heaven and earth, simultaneously guarantees the acceptance of the money.
This is a fake Tu Duc silver bar due to incorrect inscription on the side right of the obverse (the last picture right). This is from the same person who cast a fake Tu Duc in Zeno # 154966. The inscription on the Tu Duc fake is completely wrong because it is copied from a Gia Long silver bar. This bar has to be fake because the reverse and sides match the Tu Duc fake (Zeno # 154966). These are probably Vietnam War era fakes.
Side A has an incorrect inscription. The inscription on this fake is from Gia Long silver bars as seen in the Annam Etudes Numismatiques by Albert Schroeder book:
http://www.zeno.ru/showphoto.php?photo=820
see # 120
Here is an example of a Tu Duc silver bar also in the Schroeder book. Tu Duc bars also exist without inscriptions on their sides (according to Bernard J. Permar's book Albert Schoeder's Gold and Silver Coins of Annam).).
http://www.zeno.ru/showphoto.php?photo=793
see # 320
The monetary value of one lang of silver on the side. On the Gia Long, Thiêu Tri and Tu Duc, the value is always the same ! : "two guan and eight mach" (1680 van). But it is well know that during the reign of Tu Duc, the silver lang had the value of 4302 van.