Newfoundland issued several overprinted airmail stamps that served as postage on pioneer flights, which are of great interest to Aviation topicalists and include some of the great rarities of Aviation Philately. The rarities are unaffordable for the vast majority of collectors, but the lesser flight overprints are also quite scarce and worthy of consideration as investments.
In 1919, Newfoundland surcharged 6,786 15c Seal stamps (Scott #70) from its 1897 Pictorial series, increasing the stamp's value to $1. The stamp (Scott #C2) was used on covers which were carried on the first successful non-stop trans-Atlantic flight. Scott ' 13 prices the unused stamp at $225.00 ($375.- for NH).
As with many overprinted stamps, varieties were produced, including one without comma after "Post" (pictured; Scott C2a- CV = $240.- , 2,796 issued), and a much scarcer stamp without the period after "Dollar" (Scott C2b- CV= $ 450.- , 398 issued). I recommend purchase of any of these stamps in VF NH or LH condition, or on a Flight cover. C2b probably offers the most bang for the buck as an investment, given its current scarcity and market value. As with all valuable overprinted stamps, expertization is strongly recommended.
Those interested in becoming part of an international community of stamp collectors, dealers, and investors are welcome to join the "Stampselectors" group at Facebook. The group hosts lively discussions concerning stamp investment and practical aspects of collecting, as is also an excellent venue for those who wish to buy, sell, or trade stamps.