The latest addition to my collection is this quarter sheet of overprinted stamps.
This is a high quality scan so you can zoom in a bit and check all the details. The stamp itself is quite common but getting large multiples is certainly not. Since the overprints are made using hand devices there is a lot of variation. This often makes checking the overprints difficult because it is necessary to distinguish between variations resulting of the so called “human factor” (including changes in the ink, pressure and angle used when applying the hand stamp and how often the clerk used the hand stamp before he used fresh ink) and variations that mean the overprint is forged. With time and training and a lot of material for comparison most forgeries can be detected by checking distinctive regions of interest.
Here some examples of the variations. Some of them are useful for forgery detection.
Example 1: The serif is quite prominent and in the shape of a diamond. 
Example 2: The serif now looks like a triangular hook.
Example 3: The diagonal line is broken. Not a distinctive characteristic of a genuine overprint but it shows how far the variation can go. For forgery detection the shape of the “handles” in the middle is quite important.
Example 4: A left over of the frame. The unframed Z overprint are the successors of the framed Zs when the frame was removed due to wear.
Example 5: A very prominent leftover of the frame at the bottom of the overprint.