The early Sandcast backing plate was made using a sandcast process with later plates being diecast. Don't know at what frame number these early brake plate changed to diecast type. Even though the response when asking owners to check what they have is either very low or non-existent, I'll ask again. When you next have your rear wheel off or you are able to check, can you respond here and advise what type you have and your frame number. The attached photo shows the two types. Thanks