Someone had a stove vented through the bottom hole (into a bricked up fireplace??) & if your house was ever slum-lorded like mine, when the soot filled up the cavity, they closed the first hole & made another opening higher up in the flue.

The bricks above and below the mantle are similar in appearance, which doesn't seem to support the theory of a fireplace here having been bricked over. And we both had our hands in that bottom hole, there is no soot.

If there were issues with the true chimney, could they have run a true old-fashioned stove pipe up through there. If there any indication that a flue for a stove pipe was ever run through the roof?

I guess my question is this: there appear to be two parallel flues. What was the purpose of the left (upstairs) flue since there is no sign of a firebox? And was the mantle always just decorative?

Well, forget the slum lord bit. We have the same set up in our 1st floor kitchen (see photo) and the 2nd floor above it had a blocked opening. I seriously doubt anything nefarious. In our 1st floor kitchen the chimney was obviously used for the cooking stove. 2nd floor -who knows? But certain that it was for heating, and also certain that not every room received a fireplace ($$), -some just got a wood stove.

Stranger is that you have (had) lath covering the chimney, which would be a fire hazard if a device was ducted there. But if one was once and later removed, one would think that this would have occurred after the period when wooden lath was used (given your house @ 1919).

Oops, here is the photo?

Thanks for the photo, but I don't know what I am supposed to be seeing in it. Is it a double flue? Where is the opening? Is it just a small hole to vent a stove?

In my neighbor's case (and mine), there is a bedroom above a dining room. Both of our dining rooms and our bedrooms have mantels.

The easiest way to see how many flues you have is to go on the roof & look down the chimney, assuming the dividing partitions are still in place.

You had chimneys for wood or coal parlor stoves with decorative mantels added to make it more upscale in appearance. My house has the same setup.
One flue serving stoves on two floors. Not up to present code; they are not even lined with tiles.
Casey

That explains it - parlor stoves. Thank you Casey and everyone else who helped walk me through this.

My chimneys from 1908 are lined with flue tiles (original), one served the coal furnace and a gas fireplace on the first floor, and there was a hole for a gas heater in the bedroom directly above--only the parlor has a mantel and firebox.

The second chimney was for the kitchen stove, and served two heaters on the second floor, one in the bath, the other in the second bedroom...neither had a mantel or firebox.

I don't see any indication in the brickwork that there was ever a fireplace, just a heating stove.