First, I enlarged all the photos and really checked them out,
, let's start with the biggest elephant in the room. A center chimney foundation and first floor joist system that does not support there ever being a third firebox. One might take a stab at the fact that maybe that section of foundation masonry had been removed, but then the framing, as you say, shows signs of being all original including the sub flooring nailed to it. Correct? The two parlor brick hearths seem to be original to the house. Are the built in cupboards on the side of each parlor firebox original also? Normally they would have been on the other side of the firebox, in both rooms. So clue two is that if they have always been there then I would assume, pretty strongly that there was never a 'traditional' 'keepimg room' firebox.
So, then we look at two things. Is there any sign that there was a stove pipe hole into the stack where you would have thought the missing firebox would have been?
Second, are there any indications that the ell was original or added later?
First job. Measure the length of the bricks being used in the other two hearths. Then measure the length of the bricks in the ell on the facade of that ell firebox , not the hearth as I strongly suspect that the hearth ones are newer brick. The facade ones look to be early brick and nice tight early mortar with the correct color. (you can compare that also, the spacing in between bricks)
That yellow woodwork is suspect JEREMY,
. Suspect to be very old! Maybe not, as I am only going on enlarged grainy low pixel pics, but I see things in it that make me feel its quite old. Even that cupboard, at least the doors, looks to be old. Hinges NO. But as I say I can't really see it clearly. What does it look like inside there? How about thos drawers? How are the built? Maybe you already know it's obviously new?
I'm guessing, and yes it's way to early for a guess, that the ell was built the same time as the main house and always intended as the kitchen. Most all "added on" ells became the kitchen in earlier homes.
Is that stack in the ell attic for that cooking firebox we are talking about now? If so, measure that brick also. Being you can maybe find a length and width
with those ones, then also get the width of the parlor hearth ones. We are trying to see if the masonry was laid at the same time.
Lets start with this. Bed time! soryy,