Op, I’ve been in the same boat.
I was a SAHM. I signed up for child benefit, to get the NI contribution/ state pension stamp, but opted out of the money, as I’d have to pay it back, as DH earns too much. The month my youngest DC turned 12, was when this stopped.
As it happened, I was working 1 day a week until then, and I started paying NI myself the following month when I upped my days to 3 a week. I thought there was a 1 month gap between this and I could have paid up 1 month to make it a full year. Turns out I don’t need to as I did extra shifts that month.
if you earn £120-180 a week, your NI is paid for you, or you don’t pay it, and you get your NI/ pension contribution paid for. Over this, you pay NI.
I have a note in my diary to call up the pensions dept. or whatever relevant place is, to make sure my last year is all up to date.
Well done on doing this. Many of my friends haven’t and won’t qualify for a state pension They are all adamant that they get a state pension anyway because they have British nationality and think you don’t have to contribute to it. Madness.