Help replacing 870 Express rear sight part

I have a 870 Express barrel with rifle sights but the blade of the rear sight has been lost. So the rear sight ramp is still there and in the barrel, but I don't have the notched piece which rides the ramp.

It's a small part that shouldn't cost much to replace, but I don't know where to find it, what to search for, what exactly to call it, etc.

Looking for rear sights I get complete sets that need me to drill and tap the barrel or receiver... or complete barrels, etc.

Can anyone help me find a link to the part, a part number, or what exactly to call the part in order to search for it? Thanks!

moon928, thank you! The second one is exactly it.