What is this on the sides of it... looks like it can be unscrewed? Other than that is could be something that was eliminated and capped. Hard to say from the picture... what is on the other side is that the side of the house or on the same wall where the sewer pipe exits the house?

So the main sewer line runs the length of basement and exits through front (to public sewer) but it also exits through back wall, which is where, yes, it looks like something was capped. it looks like areas around cap and bottom of pipe that exits through the back are leaking. House was built in 1910. Is it possible that main line running out back wall was never capped? That waste is flowing both ways?

In the back, it could be what it was an old sewer exit and this was a vent if it's directly above the sewer pipe. The rear sewer main this was connected to must have been eliminated and the sewer line was also capped after it was diverted to the front.

In addition, maybe at one point and time, there was another structure in the back of the house that had a bathroom in it and it was connected to the house sewer and after it was removed it was capped.

Most likely it's the 1st scenario.

Give a local plumbing contractor a call have him come out, most will not charge you for this and ask them what it is, and if you just moved into this house you will kill two birds at the same time, you get to meet a local plumber that you can use when needed and find out what that is.

Good luck