That's not how classification works. Stamp or no, if the information contained within is considered classified, the format that it exists in is classified. ...
Back in the 1980s (or maybe late 1970s), there was a really fun example of this that appeared in lots of news sources. It seems that the DoD got curious about what could be learned about the US military forces from publicly-available sources. So they gave a grant to a couple of college profs to run a study of the topic. They (or rather, their grad students ;-) dug through lots of local newspapers and other public info sources for mentions of the US military, and after some months, submitted their report to the DoD. Within 24 hours, it was classified (Secret, as I recall).
Everyone who read about this got a good laugh, of course, and we all had fun mocking the idiots in the military security agencies that they would respond like this. But among the jokes, there were occasional mentions of the lesson we all might learn from this: Not being part of the government, not having access to any classified info, etc., isn't protection. We are all told repeatedly that for our democratic government to work, we all should keep up an interest in its activities, pay attention to what's going on, etc. But if we do so, our personal piles of (partly read ;-) newspapers, journals, and assorted articles from other sources could easily fall into the same pitfall that this study did. We could easily be in possession of classified information without knowing. If a security agency finds out, we could be in serious legal trouble.
Various commenters at the time suggested that we should be constantly purging our own piles of data of anything that might be related to our government. It's not enough to just ignore it and not read those stories. If we have them in our possession, we could be found guilty of unauthorized possession of classified information from the aggregation of our information.
Or we could just not have any information of any sort among our personal artifacts. Don't even subscribe to anything that might contain information. (Writers usually suggested a few periodicals that should be information-free, such as People, Sports Illustrated, US News & World Report, etc. Nowadays, they'd probably list CNN and Fox News. ;-)
Unfortunately, I've forgotten the names of those researchers. It might be fun to find the reports and read them again. Google doesn't seem to know about them (or I just didn't guess the right keywords). Does anyone here remember that news story? Do you know how to find it again?