Between January 1, 1857 and June 24, 1947, hundreds of Newfoundland stamps were produced
These stamps were used for letters until April 01,1949
They were
not demonetized when Newfoundland left the British Colony to become part of Canada.
What does demonetized mean.."
deprive (a coin, stamp or precious metal) of its status".That means after Newfoundland entered Canada it could still use pre Canada Newfoundland stamps as payment on a letter just as though a Canadian stamp was used.
Below are 3 examples of Newfoundland stamps used after becoming part of Canada.
Here is a letter sent from Topsail in Conception Bay South, Newfoundland...It used a Canada Scott 479 and to make up the postal rate for that year, some one also put on a Newfoundland Scott 233. This cover was canceled April.24.1969
The Scott 233 (NFLD) hs a perf.13.7 x 13.7
Below is another cover With Canada Scott 401 with a Newfoundland Scott 255 to make up the postal requirement for the period.
Cancel on this cover is from Topsail, Newfoundland canceled
March.20.1968 
Here is the last cover again from Topsail Newfoundland...Canceled
November.19.1966 ..Is used 3 Newfoundland Scott 233.and a Canada Scott 392 with 2 hidden Canada Scott 337 under the Scott 392...Guess he needed to add more to make up the current postal rate.
NFLD Scott 233 stamps are perf. 13.7 x 13.7 and the bottom right Scott 233 is perf. 14.1 x 14.1

BOTTOM LINE..Anyone can say use a NFLD Scott 1 or any other stamp to mail a letter...not advisable..haha..but legal.
Robert