Tampering
Differences between mint and used values for the same stamp have led to a small industry in removing, or adding, postal cancellations to stamps. Another common practice is to attempt to remove fiscal cancellations, which are often pen cancels, in order to change a stamp used fiscally (for tax purposes) to one apparently unused. Stamps available for both postage and revenue (fiscal) purposes are usually worth more unused or with a postal cancellation. Madame Joseph specialised in the addition of forged cancels to stamps that were worth more used than unused.
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