For the benefit of
non-stamp collectors, the philately fraternity call these "fake"
one-off stamps "cinderellas".
The first issue of
Dan Dare stamps was produced in the UK in 1953 as a publicity
item for Lever Brothers' Lifebuoy Soap. There were 32 stamps in
total and were issued in groups of two in cellophane packets inside
the wrapping of the soap. A special folder was also issued in
which to place your stamps, and a special "Letter from Digby"
explaining all this was also given away.