It is well known that the mint mark is a brand.
Here the two words “mint mark” are used instead of someone's fantasy “mintmark” used from time time in Numista catalog. This misleading is not the case of the article.
For it is a brand the mints are ready to show it on their production and sometimes even borrow it to other mints.
Heaton mint has its own mark “H.” This mark may not appear on coins struck for territories which are not considered to be part of Crown, or formally of the Realms.
But the Prince Island coinages seems as an exception.
For example it looks like a 1 cent coin of 1871 does not bear any sign of a mint. See images
Despite of that it was struck in Birmingham on the Heaton mint, the mint did not mark its production at all. At the same time coins struck there for the whole British Realm from Australia to Canada always bear mint mark, including such as Newfoundland or dominion Canada itself.
Could it be that reasons for omitting a mint mark “H” are known?