|1910 (or thereabouts) saw the issue of
two new overprints - a 2½r giraffe value from the 1901 issue surcharged and overprinted with "5 REIS"
and "PROVISORIO", and a 100r camel value from the 1901 issue
surcharged and overprinted with "50 REIS" and "PROVISORIO".
The reasons put forward for this vary. In a contemporary article (which can be read here) Betram Poole refers to these two stamps as the 5th Issue (with the 1901 issue as the 3rd issue). Poole states the official explanation for these stamps, that they were printed to counteract shortages of the 5r and 50r values, but poses the obvious question that since the overprinting was done in London (it is done in the same style as the London overprints of the 1903 issue) then why didn't the printers simply print new 5r and 50r values, rather than go to the trouble of overprinting existing values?
Poole puts forward the view that when the 15r and 25r giraffe values were overprinted with "PROVISORIO" in 1903, some of the 2½r and 100r values were also overprinted in the same way - in the opinion of the author of the article, the Nyassa Company refused to accept them (understandably, since it hadn't ordered them), and the printers were therefore saddled with them. When the shortage of 5r and 50r values arose, therefore, the printers agreed to surcharge the 2½r "PROVISORIO" with its new value of 5r, and the 100r "PROVISORIO" with its new value of 50r, in order to use up otherwise useless and superfluous stock.
Claude Emerson, in a more recent article (which can be read here), tends to a similar view, agreeing that the stamps were never ordered by the Nyassa Company, but were printed by Waterlows and subsequently accepted by the Nyassa Company.
John Dahl's view is that the truth of the matter is much simpler than that - the stamps were dreamt up by Waterlows, the printers, to take advantage of the buoyant philatelic market for Nyassa's stamps at that time. They were never sent out to Nyassa for use in the postal system, and all "used" examples around today were cancelled to order in huge quantities by Waterlows with a light blue cancel and released direct to the market.
Further supplies of the two stamps were printed sometime between 1921 and 1925 to satisfy collectors' demands. Afinsa says 1921, as does Scott. Emerson says 1923 - 1925, but his editor indicates that the Simoes Ferreira catalogue says 1921.
Whatever their date, stamps from the reprint can be distinguished from the originals by the distance between the two lines of the overprinting (ie. the width of the gap between the two lines) - on the original giraffe value the two lines are 1.4mm apart (some catalogues say 1.5mm), whereas on the reprint they are 2mm apart. On the original camel value the two lines are 1.6mm apart (some catalogues say 1.5mm), whereas on the reprint they are 1.2mm apart.
Errors, misprints, inverts, forgeries, etc...
There exists an "error" on the 100r value where the surcharge of "50 REIS" is omitted, but when one considers the spurious nature of the stamps anyway, this would seem to be just another of Waterlows' tricks to part the collector from his cash. Given that it catalogues at around £240.00 mint, the trick seems to be working. Betram H. W. Poole's contemporary article (here) mentions a sheet of the 100r with the top row of the sheet surcharged "PROVISORIO" only, covered in more detail in Claude Emerson's article (here).
Afinsa nos. 50 and 51.
The 1910 overprints catalogue at £1.20 each for mint examples. The reprints aren't recognised by all the catalogues, but in those that do there is a large discrepancy in values - Scott values them at just 25c, whereas Yvert, for some unknown reason, values them at more than the 1910 printing - around £25.00 each, mint.
My inclination would be to believe the lower value, as given Waterlows' propensity for such things there is no doubt that the reprints would have been printed in substantially greater quantities than the 1910 printing.
The 2½r overprint