[Dustydoo]

Is the hole in the cap threaded?
If so, not sure if it is original, only thing I can think it would be for is a sling.
That weight looks homemade, a
I would guess it goes in with the magazine spring. Like a 2 shell plug in a field gun

 

[skeet_man]

If you drop it in the magazine tube and it's OD is smaller than the ID of the spring, it's just going to slide back and forth, and may end up having a slide hammer effect on the magazine tube threads or the follower and barrel support. If you can't figure out how to attach it to something, you should leave it out.

The cap screw may be there to attach a washer to (that's missing) to prevent this from happening...

 

[entropy]

The hole in the mag cap has two uses; draining water should the gun get dunked (they are duck guns, first and foremost), and for attaching an Uncle Mike's sling swivel to the cap. They are not threaded; the swivel has a post that holds a circlip on the back side of the cap.
I believe skeet_man is right; get a washer that is wider than the spring and screw it to the top of the weight with the Allen screw.

 

[charleyj10]

Do you have any idea of the age of the gun?

 

[6541]

I attached an Uncle Mike's sling swivel, like entropy observed, through the hole in the magazine cap in an 870 field grade gun. As memory serves, I may have expanded the hole with a drill to allow the attachment of the sling swivel.

I bought the 870 in 1965 and it came with an 8 oz. weight that fitted into the magazine tube behind the cap and was held in place by the magazine spring.

 

[bossbasl]

Highly unlikely weight is original Remington made.

 

[nitro27man]

First, the weight is not a Remington factory option or part.
Second, the magazine cap has a hole to attach a sling swivel.

Scott Hanes

 

[dagnarble]

Is the end of the weight threaded? Maybe you attach it to the cap