What was the reason for the 3d and 6d overprints (Scott 94/95, Gibbons 159/162)? It doesn't make sense to overprint the stamp with the same value, so I assume it was a surcharge of some kind?
My search skills must be failing as I could not find explanation online or even in the Gibbons QV Specialised catalog.
When these were produced there was a lot of official concern about losing revenue through the re-use of cleaned stamps.
The suggestion was made to print the current range of stamps all in the same fugitive lilac colour and overprint them with these large figures to make them easier to distinguish.
In the end only the 3d and 6d were produced this way and these were soon replaced by the unified lilac and green series which also used fugitive inks.
Here's a nice description from the GB Overprints Society site:
