When, in April 1882, Revenue stamps were authorised for postal purposes, the temptation of financial gain in producing postally used-looking examples (which commanded significantly higher prices) became too much for certain enterprising individials. Many examples exist of Revenue Stamps which have been cleaned of their manuscript cancellations and postmarked with postal-type cancellations in an attempt to command a higher price for the stamp. This is potentially a lucrative business especially if you can dupe novice collectors – as we all were once!
Another area where the genuineness of a postally used revenue stamp comes into doubt is from the period 1870 to 1910 where postal-type cancellations (Usually A-class, F-class and H-class) were used to cancel revenue stamps, especially in the larger centres, with no supporting defacing evident (eg foil slits, embossing, manucsript endosement & pinholes) thus making it nigh on impossible to determine the actual use of the stamp, and thus the price it commands. As long as the postmark is after April 1882 (when revenue stamps were authorised for postal use to cover a delay in delivery of the 1882 Second Sideface issue) it is exceptionally hard to tell. I guess as far as the clerks of this period were concerned once a stamp had been cancelled that was all there was to it – this does not however make it easy for the revenue collectors in the current century!
The only way to genuinely confirm a revenue stamp has been postally used is for it to be either on a large enough piece or cover to confirm its postal use. These examples are indeed few and far-between.