Checking my stamps, I have found this USA Scott #1019 which has hollow letters instead of solid letters ("Centennial", "Washington" and "1953"). Scott lists no varieties for this stamp and I have not been able to find it searching this forum and google. All instances I found are with solid letters. For comparison, please see the "normal" solid letters stamp above and the "hollow letters" below. I also attach a detail pic.
I would not say it's an under-inked print... Is this a variety? A printing error? I would certainly appreciate your opinions. Thanks!