I've got a Marlin 1889, which has been in my family for many years. It is fitted with a folding tang sight and folding front sight (kind of a ring with a vertical pin inside)....not sure if these are original equipment or later modifications, but I'm ok with them. If they are modifications, they were done long, long ago. However, the sight which SHOULD have been at the rear of the barrel is missing, just an empty dovetail there.
My question is: What was original equipment rear sight on a Marlin 1889, and where might I find it?

It must have been taken off because it interferred with the tang sight. Could be tricky to find.

Sometimes you will find stuff like that on ebay or antique gun parts stores-I cannot think of one right now.

I had an old Winchester of my Grandfathers, the original sights showed up in the bottom of a box of his stuff at my parents-I knew what they were right away.