Unfortunately, a gauge block or standard would have not caught this odd error unless I knew exactly at what dimension to check with a large gauge block set.... it only showed up at certain rotation angles of the needle. At other needle angles, the caliper is still quite accurate. I only caught this as I suspected the reading to be in error, and compared it to other devices that are not subject to the gear wear that dial calipers are. It was surprising that the error went from under .001" to .0025" in a few 10's of thousandths of, but if a gear tooth gets worn or damaged, then the expected result would be 'localized' errors.
Already have a new Mitutoyo here....(The 6" Starretts look to be made in China.) This one is probably 40-50 years old so it will just get consigned to work needing low accuracy.