Hi Wessel,
looking to the markings of the hindwings all should be males.Males in Danaidae have different markings on the wings,sometimes difficult to see.In this species I found in my collection apart of the mentioned markings something else.Just turn around your butterflies.
1.

T.septentrionis,male,hindwing verso
2.

male,external sex
3.

T.septentrionis,female,hindwing verso
4.

female,external sex
The external sex organs all look similar in Danaidae.
Regards
Hans