Tin can clock weights were the typical and standard weights found in production tall case clocks. Weights were sent out empty and those who purchased the clocks filled the weights themselves. Original weights can be time capsules from the past, sometimes filled with broken Dutch oven or tea kettle fragments, miscellaneous scrap metal collected at blacksmith shops (broken chains, hinges, bolts, etc.), or even old lead typeface. 
In order to save myself, and the buyer shipping costs, these cans are shipped empty. You can fill them with whatever you’d like, but it’s important that they are not too heavy. 
The strike side weight should be around 3 pounds. The time side should be 3 1/2 pounds, 4 at the maximum. 
You are buying two weight shells.