just because it's stamped that doesn't mean that it is that purity. need to test it, especially since the seller and provenance and now the mine or assay logo cannot even be determined by someone from one of the largest, oldest and most widely known assayers in the world.
With a little research you should be able to find out how many cubic inches of pure silver would weigh 100 troy ounces (8.857 pounds av. or 3.11 kg) and compare the dimensions to your bar. BTW have you weighed it on an accurate scale? The weight adjustment to correct for the .001% of impurities should be virtually indetectable. My guess? Lead would be a closer match.
Once again, this bar belongs to my friend in another state, so I don't have physical access to it. I will pass that along to him and let you know what he finds on the weight and purity test. Where can you get something like that done? Weight can probably be found at a local coin shop but where can I send him to get the purity tested?
buy a test kit and do the purity test yourself, see the link i posted earlier, that's a link to an eBay seller who sells the test chems you'd need