Your gun is either a Police Magnum or an Express Magnum. There's no such thing as a "Police Express".
From the pictures, you have a genuine 870P Police Magnum gun.
For some years now the Police guns have had the Improved Cylinder choke.
This year, Remington appears to have gone to the Improved for all their Express 18" guns too.
The slightly tighter choke gives better performance with buckshot and slugs.
Look on the left rear of the barrel, and you'll see the stamp IMP CYL, which means Improved Cylinder.
I recommend measuring your barrel.
Close the bolt and insert a rod down the bore until it touches the front of the bolt.
Mark the rod at the muzzle, remove and measure.
For many years, Remington has used either a true 18" or 20" barrel on the Police and Express guns.
It appears that they've recently gone to a 18 1/2" barrel, which makes it the same length as the extension.