From what I can make of it, your interior walls are built up as follows, as it appears in your text:
1st layer - Wooden lath or split lath applied over the timber frame. (Horizontally oriented?)
2nd layer - An interior smoothing surface of 1 inch square edged lumber.
3rd layer ( we are yet apply any plaster. Right?) — Vertical strips... for keys.
4th layer - Plaster mixed with hair.
The text doesn't specify any further layers but since it does say 3 layers of plaster I guess there would be layers 5 and 6 of different or similar plaster mixtures and surface textures.

And this is plastering the infill panels I'm guessing because this is in the hewing questionnaire, right? And we aren't going to plaster over those fine timbers now. Or has that subject title totally lost any meaning at this point?

This subject caught my eye just because the other day I had to climb up in the attic to track down a leak when the ice and snow on the roof suddenly melted. While up there I noticed a wall I had plastered a few years back with — let's see, how do you call it, well, I'll say clay, sand, and cow manure, horse and reindeer hair mixed up good, and how good it seemed to have dried up and become solid. And largely thanks to the cow shit. Then I made a picture.

Someone looking good at those keys would notice a sort of grayish brown and also the darker brown. The darker color is the result of throwing the poop in there and has the effect of really binding the plaster well.

It's a foul smelling business at first but pretty soon the offense fades away.

I guess the only thing I would add to the other entry up there is that the type of hair could be of importance as some — human, dog, cow...— are normally quite fatty by nature and can better be substituted with hair known not to have this characteristic.
And it is true or the truth, that there is no beating the appearance of a good old-fashioned plastered wall surface whatever form it may take.