Hi, NBStamper is right, if you find (unlikely), an 2d over print with stars in the right-hand bottom corner, you're onto a winner. My 2008 catalogue values it at NZ$525 although you could possible obtain one for a little less if you looked around. These are some from two old (1966) albums that I have - unfortunately the expensive ones missing.


Overprinting is quite common on quite a number of NZ stamps. Here's a couple of examples.
