New rusty screws for scope.

Today I tried looking for some replacement screws at a place that carries anything and everything dealing with hardware. They could not find one screw to replace the scope's old ones. All the sizes were off by millimeters. How could this be? Has screw technology changed in 20 years?! Alas, I was told that the screws were usable and that I should purchase a rust remover of some sort to bring them back to life.