According to Lillian Faderman in Odd Girls and Twilight Lovers, wartime in the U.S. offered a more tolerant atmosphere for lesbians, especially in the military. This was in contrast to the 1930s, when a greater awareness of the existence of homosexuality (no longer did the true relationship of two close female friends go unsuspected) ratcheted up hostility towards same-sex partnerships. In The Night Watch, we get a sense of the war years as a time for boundaries to be crossed, including extramarital affairs and areas where lesbians could exist with some amount of freedom, like working the ambulance "night watch." (For me, though, one of the gaps in the story was how exactly Kay and Helen got together -- Helen never having been with a woman before, and there being no easy way for Kay to figure out which "sort of love" she gravitated towards.)
While Helen lives in fear of being found out, it is still relatively easy to speak innocently to her coworker about the "friend" she lives with, and even her and Julia's vulgar neighbor doesn't speculate salaciously about the nature of their shared living arrangement -- the "eunuchs upstairs," he says about them.
The all-female representations of the universally human pathos of relationships -- for example, being in love with someone who doesn't love you, as Kay experiences with Helen, as Helen ends up doing, and as, we eventually learn, Julia has done with Kay -- make this a book that, indeed, writes lesbians back into history but avoids being of niche interest.
What do you think Kay's future holds? Mickey's? As their 46-year-old friend Binkie says, "'Tell me truly: doesn't the life we lead ever get you down? It's all right when one is young. It's positively thrilling when one is twenty! [...] But one gets to an age where one sees the truth of it. One gets to an age where one is simply exhausted. And one realises one has finished with the whole damn game...You wait till you're my age [...] and wake every morning to gaze on the vast tract of uncreased linen that is the other side of the divan. Try being gallant to that...We shan't even have children, don't forget, to look after us in our old age.'"
Kay, after all, has no financial obligation to work, so what will she do now? Will the return of her ring, and with it the reminder of an intense time long gone, spur her to action of a more fulfilling sort than "getting up a girl" in movie theaters?