Looks like wallpaper to me :)

I prefer repairs that resemble the original install method but you can also look into specific repair systems with plaster washers, or even use drywall and mud.

You'll want to remove all the paper (steamer, or whatever method work ok), and anything loose. A lathe/plaster wall relies on the mechanical hold of "keys" through the lathe. Basically, the scratch coat is pushed through the lathe, and those blobs harden and hold it in place. So, you'll need to remove anything that is cracked and the surrounding area. You'll know you can stop removing once you see the keys through the lathe (ie ones that haven't broken off).

The actual repair would be a pretty long post, but your basic options for further googling are:

1. scratch/brown/finish, known as a 3-coat plaster (probably what you have already). Products aren't usually carried at big-box stores, so call around to plaster suppliers or masonry/stucco suppliers. They'll be able to help you with product selection.

2. Drywall patch - a sort of 2-coat plaster system. Cut a drywall patch to fit the new hole (not too thick, might only need 1/4" drywall). Use durabond to mud it in. Finish coat with durabond.

3. Look into repairing with plaster washers (I'm not familiar myself).

Thanks for the feedback! I’ll explore recommendation #1 further as best I can (there are surprisingly few plaster experts here in CO, or at least quite a bit fewer than I would have expected).

Re: the wallpaper, I should have been more descriptive. The paper has no print or pattern as I would associate with traditional wallpaper, and I *thought* I recalled having read about a paper product sometimes used in this era over plaster strictly to provide a smooth painting surface. I may be showing my great ignorance here re: historic wall trearments, but I wasn’t sure if it warranted different treatment or if I should expect to find something else below if/when removed. All other plaster walls appear to have been painted on directly.

It could be lining paper, which can be painted over, or it can also provide a smooth wall for wallpaper. However, your wall looks to be in not great condition, at least in that one spot. I am wondering if it was used to cover a multitude of cracks similar to that. There is a thicker lining paper called 2000 grade that is used to cover cracks, like you have.

Look into Big Wally's Plaster Magic to repair cracks and plaster that has broken keys and is pulling away from the lathe. Good system: used it on one house, about to use it on another.

A $50 wallpaper steamer should take the paper off pretty quick. Scrape the loose stuff with a wide putty knife, then steam off the remainder.

It certainly looks like wallpaper or wall-liner that has been painted over. Generally, wallpaper steamers are less effective with painted-over paper because the paint blocks the steam from getting to the paper. However, it's worth a try.

What we have found to be more effective is to first sand the paint to create openings into which the steam can penetrate, then steam. It's a mess, but less of a mess than any effective alternative that we know about.

To reattach the plaster and permanently repair any cracks, Big Walley's works, but it is expensive for what you get. Our method is similar but much less expensive. See Easy Fix for Cracked Plaster Walls (and Ceilings).