Eraser Wrapper!

Yes, you read that correctly! An eraser wrapper:

The wrapper used to cover the stock standard, run-of-the-mill erasers used in my childhood. 

Every kid had one: small, in bright colours, efficient standard classroom gear and made in China (Shanghai, China, I think). 

Completely forgettable except that, when you move to somewhere that doesn't sell these or anything like it, its (nostalgic/ memory) value goes up enormously. 

And so I kept my last wrapper for a couple of decades!