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did you try to use some cleaner on those before you took that pic??some of those look like lines created from pulling the drum off the brake shoes,how do the shoes look??
Sometimes the drums crack from repeated heat cycles over time. Replace the drums with decent replacement parts and your rolling down the road with rear brakes. It would be a good idea to replace or at least free up and grease the adjuster on your rear brakes while you are at it.