These two parts are the ones you read and hear about for up for upgrading the 870 Express or Tactical Models to the same parts as the 870 Police. In my opinion these are mostly feel good parts and Marketing by Remington. The Black spring is a Model 1100 Carrier Latch Spring and is used only in the 870 Police as the carrier dog follower spring. I would challenge anyone to know the difference it I hand them two trigger plate assemblies and ask them to work the carrier and tell which one has this spring. Yes, it is slightly heavier and back in the day Remington made two carrier dog follower springs, a standard one and a heavy one. About eight years back they discounted the standard one and all the 870's come with the heavy one, except of course the Police which comes with the black one.
The extractor is the Solid steel and not the MIM which is also called the Powder Metal as MIM stands for Machine Injected Molding. The solid steel one only comes on the Wingmaster and the Police. So yes, it can be considered an upgrade for your Express or Tactical Model. These below two videos show how easy it is to change out these two parts.