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I believe that with the new salt and pepper shakers, the salt shaker is the one with 3 holes, while the pepper goes in the one with 2 holes. Salt is used more than pepper anyway. Pepper should only ever be fresh ground at the table, meaning salt shakers win out.
Make sure this is not fire brick or a chimney flue. I would drill through one of the voids of the block. A block will have three webs, one near each end and the other in the middle. My DeWalt 14.4 hammerdrill has drilled 1 holes through block. I would measure the distance by going to truck stops and... funny.