A few months back I tore down the wall between the study and foyer. This wall was built in 1955 when the house was converted to apartments. I've been puzzling over this wall for months and I believe I finally have the answer.
There was a wall in the same location when the house was originally built. Sometime in the 30's there was a major renovation on the house. The wall was removed during this time. Also during this time a larger front doorway was installed. The door always looked to me like it was from the 30's and not original. The entire doorway is 7' wide, the original hallway was 6' wide. The wall was removed, the new doorway added, and on the opposite wall a built-in bookcase was added. I'm not really sure why they removed the wall in the 30's remodel, I'm guessing they wanted a combination foyer/sitting room.
In 1955 when the house was being converted to apartments they needed to put the wall back up but because the doorway was now wider it couldn't be installed in it's original location. To make matters worse, the built-in on the opposite wall wouldn't allow for the wall to be shifted over the foot it needed to go. What they came up with was to build a slanted wall. It covered one edge of the doorway on one side, and the edge of the built-in on the other.
Here are 2 pictures of what the wall looked like when we found the house(it's on the left in the first pic, right in the second):
View from rear hall
View from front door
You can see how slanted it is and how it covers up the door frame. So last December I got tired of looking at the crooked wall and tore it down.



I then carefully removed the built-in. I will be installing it on the wall under the stairs eventually.

Over the weekend I started the project of building the new wall. First I had to cover up the space where the built-in was. I toyed with a lot of ideas for the space but eventually decided to just cover it up for now since I'm running short on time.
First I installed some studs using some of my salvaged 2x4's from other parts of the house. I'll be using my salvaged beadboard for the bottom 4' of the wall so I needed to come up with some type of sub-wall to nail it to. The lath is 3/8's thick, the plaster is another 3/8's, then there's 1/2" drywall above the plaster. The yellow pine floorboards that I have are 3/4" thick. That is the same thickness as the plaster and lath together which brings me exactly to the depth I need.

With that out of the way I could begin rebuilding the main wall. The first step was to cut back the outer layer of drywall about a foot all the way around. In the picture below you can see where the original wall was and the foot of drywall I cut back:

Next I laid out my new wall on the floor with chalk-line. the new wall will have a double door opening in the center of it for my salvaged french doors.

I then framed out the rest of the wall, I used most of the same studs that were in the wall I took down. With the wall up I built the door jambs and hung the french doors. I had to use new wood for most of the jambs as I didnt have any salvaged pieces tall enough. These doors are really tall, almost 90".
View from back hall
View from Parlor
View from inside the Study
Once the wall is finished I'll strip and stain the doors and trim.
Here's the door hardware I'll be using for the door:

Once the door is refinished I'll polish it up. I'm hoping to complete the wall within the next few days.