The NSP has many departments. Some wear their uniform almost all the time, some wear it occasionally, some wear it almost never (unless they're getting a medal or attending the funeral of a comrade). As it happens, five years ago the largest uniformed department became much smaller, and also became a non-uniformed department. To the man on the street, NSP uniforms suddenly disappeared from sight.
They're not traffic cops, they're secret police.
For certain tasks, the uniform (black and imposing, I believe) is appropriate. So each officer has one, with shiny rank tabs and a polished party membership pin. They may be worn when the secret police wants to act overt and unsubtle.
But mostly overt action has been farmed out to the ordinary beat cops. There are literally millions of them, you see one at every corner and you are supposed to see one at every corner. This is a significant expense on the government payroll, possibly rivaling that that of the secret police. No need to duplicate the ordinary police by putting an unsecret policeman next to the ordinary cop. Not any more. In the early days it was different, see below.
You are not supposed to see a secret policeman at work. You are supposed to worry all the time that one may be watching you.
- That lady at the supermarket checkout, is she noticing that you buy twice as much milk as usual? Do you have a visitor from out of town who is not registered with the police?
- The janitor of your tenement block, it is almost a given that he's an informer or he wouldn't have gotten that job. Probably a veteran too old for the frontlines.
- The primary school teacher of your kids, she is noticing what they say about the Dear Leader, but is she also writing reports?
To compare your fictional totalitarians with real ones, the German Democratic Republic had a secret police with 90,000 full-time staff and 170,000 informers out of a population of 17 million. They also had an overt police with 80,000 full-time officers and 180,000 reserves.
What happened five years ago?
Historically, your totalitarians took power in a violent struggle with all other ideologies. Say your secret police grew out of a party organization, in part to bring the regular police into line. They got their (black and imposing) uniforms to trigger fear and instant obedience in the regular cops, who would then bash heads to trigger fear and headaches in the people.
At the same time, the secret police would recruit a network of informers and use plainclothes officers to manage them. Some of them were ordinary police, who took a second oath, got a second personnel file, and mostly carried on with their jobs. Others were teachers, clerks, old grandmothers.
Over the decades since the glorious revolution this informer network and their handlers grew in importance relative to the uniformed branch of the secret police. By now all regular cops are second or third generation citizens of the dystopia, they don't need a NSP officer watching over their shoulder at all times. So five years ago the cop-watching department of the NSP got lost in an administrative shuffle, by far the largest uniformed NSP department at the time. (Other uniformed NSP units include their SWAT team, some of their forensic people, and the guards at their prisons and other installations.)
Why keep this secret?
Obviously the size and disposition of the police forces, both overt and secret, are state secrets. It wouldn't do if the citizens realize that there are almost a hundred of them for each police officer.