He had these in plastic so must be important?
No. In this case, the catalog value of that Hong Kong pair is about $8 if MNH (mint never hinged). Collectors are all over the place in their philosophies about storage and display, especially the further back in time you go. Often, a better way of determining a collections value or importance is how well organized it is, but this is also not foolproof by any means. My guess is, the seller has put this pair in the card for ease of sale/shipping, and your uncle just never removed them.
Ok I got it now. If it's been used, look for light cancellation and old stamps. Overprints haven't been used. So in terms of it being worth anything of value , would these be for example?
You're close, but not quite. A cancelation only shows that the stamp has been used. An overprint serves a different purpose. There are millions of stamps that were overprinted by a government and then are used for mail. One has nothing to do with the other.
Age is not a strong determinant of value. It's a part of the equation, but many other things matter: condition, the number originally issued, variety of paper/watermark, and a host of other things. There are stamps issued in the 1960's by the PRC, a single one of which may be worth more than everything you've inherited
. In most cases, an unused stamp will be worth more than its identical-but-used cousin, but not always. Sometimes the used version is far more rare and worth far more.
All collecting hobbies share one rule: Do no harm; you're only a temporary caretaker. Most everything else is a guideline.