The International sets show a list of countries associated with
large international sets. Typically these sets have been issued to mark an important event, such as a coronation, royal wedding, olympic event and
other similar periods of interest to multiple countries around the world.
If you find it difficult to identify a stamp, try scanning it at a high resolution. This will give you more detail than using a magnifying glass.
The image below is an example of this, I could identify the stamp as Republic Dominican once it was larger.
If you do not have a scanner try photographing it.