Welcome to Numista. It is difficult to say anything for certain on a heavily circulated 50 year old coin. True mint errors of value more than likely would have been caught a very long time ago and would be documented. This could be attributed to many things. Grease blockage which is not an error,, wearing dies, which is not an error because it is inherent to the manufacturing process of all coins and they manufactured almost 254,000,000 of these in 1977. but most probably simple wear or the cumulative effects of a combination of factors. This example is otherwise unremarkable.
N#304.