Was the house important enough to have a curator? Years ago, I asked unanswerable questions at the White House of the Confederacy & the docents had the curator call me at home later that night with the explanation. And this was not a question-friendly museum so I was shocked to receive a call back. If you don't get an answer on here, that might be another avenue to pursue.

Inquiring minds want to know...........

1810 lath was likely to have been riven(split) from larger boards rather than sawn. You didn't say if it was sawn or split lath, but assuming; it's very likely for split lath to have extremely irregular surfaces, not just edges. If it is sawn, it's not 1810 stuff, but later, after the advent of the circular saw.
Casey

I am 99% sure this area had the original split lath, but the beveled cut was a clean cut, not irregualr at all. So I guess it would have been from a circular saw. Gut says it was from something later, like the Victorian era, but I can't imagine what.

antiquesilver, it was the curator who originally pointed out this mystery to the tour group! He has been in the business for over 25 years and said he's never seen anything like it.

Maybe the cut was the beginning of a project that was then abandoned for whatever reason?

Here's my ignorance showing: I'm having a hard time visualizing what you are describing. Lath to me means the thin strips over which plaster is applied, which means in a finished room it would not show. Is there another meaning that I'm not aware of? Or was the ceiling in this room never finished?

All the wood was low grade stuff ad not planned smooth.

Rough was better for plaster adhesion.

It may have started out that way and just happened to be installed at the same place.

Lathe came in large bundles of the same thickness and slightly varying with and often widely varying length for each piece.

it looked almost like scrap wood and likely often was made form smaller scraps at a sawmill).

I also do not understand how you are viewing lath in a basement ceiling.

It should be covered in plaster.

3 coat for wood lathe.
Did someone strip the plaster?

Yes, the plaster is missing from that part of the ceiling, so the lath is visible. The house is in the middle of a restoration. It looks for all the world like someone took a saw, held it at an angle, and cut down the entire length of the ceiling without removing any lath. I agree that it was for some kind of project, but what kind of project? Boy, this is going to bother me now!

I admit I too am having trouble visualizing this, but why couldn't it be as simple as whoever cut the lath had to hold the saw at an angle to avoid hitting the flooring above?

Why the cut was made is another question entirely.