Differences between the tilakkhana and the Dhamma seal I suppose that there is (are) differences between the tilakkhana (anicca; dukkha and anatta) and the so-called three dhamma-seals (anicca; anatta and nibbana). Can anyone help me?
This article, Four Dharma Seals, says:


  As suffering is not an inherent aspect of existence sometimes the second seal is omitted to make Three Dharma Seals.


From that I get the impression that,


The tilakkhaṇa is the earliest doctrine (in the Pali canon)
Later someone added nirvana to that list (making the four Dharma seals)
Then, someone decided that if nirvana is in that list, then dukkha isn't permanent/inevitable, and so they removed dukkha from the list.