The Royal Mint says about this product:
"This 500 g cast silver bar available from The Royal Mint Refinery (RMR), contains 500 g of 999.9 silver and is imprinted with a unique serial number and the RMR logo. This half-kilo (500 g) silver bar offers the most cost-effective way to purchase 500 g of silver, which offers a higher-value purchase opportunity for investors and those who wish to own and trade larger amounts of silver.
Unlike minted bars, cast bars have less intricate designs and due to the manufacturing process, there can be unevenness in the finish of the bar. This reduction in the manufacturing cost is then passed on to you, resulting in a lower premium. This finish is only aesthetic and does not affect the value."