I believe the counterstamp was a form of advertising back some time ago. This is not an error but definitely a nice collectable with some history behind it.
The 1900 I found in a 38.00 I. H. C. purchase about 4 years ago, I am still trying to complete the best set I made and have about a dozen holes to fill. The 1896 I got from jamies flea market lorain,Oh About 2 years ago.