Correct, they are the same weapon built with the same parts except a couple small ones. The only difference is the finish, the stocks and two small parts. The Express has a polymer trigger plate assembly the Police and Wingmaster still use the old cast metal.
Every other part is the same in every 870 and there are no plastic parts or beefer parts as stated in another post by someone.
There is only one 870 and the reason other models cost more is you are paying for high end stocks and finishes, not parts as these are all the same. Anyone not understanding this simply go to
www.remington.com and click on Support, when that page comes up click on parts, and then down load the parts list. for 12ga you will find only one left shell latch, one right, one hammer, one hammer spring, one hammer plunger, one disconect and so on and so on. Oh, did I mention they are all built with the same parts. So where are these other parts if not on the parts list and why are they not manufactured by Remington? Because they don't exsist except on these gun blogs by people that don't know the subject and give wrong information.
It is this simple. The Police and Wingmaster have a steel extractor and the Express a MIM extractor. The police has one small spring different then the Express or Wingmaster and that is it. All other parts are the same except for the trigger plate assembly I mentioned.
Look at the parts list and see the 18.5" Express matte finish bead sight barrel cost $134 yet the 18.5" parkerized police bead sight barrel cost $101. Yep, the Express barrel cost $33 more then the police barrel. The Police receiver is $247 and the Express receiver is $247. The Express breech bolt is $97 and the police breech bolt is $97.
Learning the parts list is the best way to understand these weapons and to avoid making incorrect comments when the subject comes up. Simply repeating something else you read on another blog is not answering the OP's questions if it is wrong.