Date of 50 naira (nigerian money) w/security thread

When did they first start putting security threads in the 50 Naira?

Without a date on my bill, how can I tell how old it is. Right now, I’m pretty sure it was printed between 1984 and now. But was hoping to get a little better time frame. It has a security thread and a water mark and looks like this. but it doesn’t have the date ‘2001’ like the picture shows.