Could it be antique heart pine? I lived in a 1900's house with it. Not as forgiving as oak, I always had to make sure there was felt or floor protectors on all our furniture, but worth it; it was lovely, and many guests would walk in, stare at the floor, and tell us how lucky we were. (Was I ever lucky in the guest department, they started me on my old house education!)

But it is much more durable than modern pine that you'd pick up at a home store and build a shelf out of, for example.

Here are some pictures to compare it to. Looks to me like you've got clear #1 quartersawn and flatsawn mixed.

Congratulations!

Here is a link that might be useful: antique heart pine

How wide are those boards?
The picture looks like our floor, and our house was built in Alabama in 1950.

Ours is 1.5" wide. It is mostly flat sawed and it is oak.
When we finish doing the big remodel jobs, we will tackle the refinishing of the floors. I'm interested in what happens with your job.

Our subfloor, laid on the diagonal, is tongue and groove not tightly placed and it is 3/4" x 3" wide pine. The roof sheathing is the same thing, tongue and groove pine. It is not heart pine. But it does not seem to have many knots, and the ones that are there are very tight knots.

Our walls are 1" thick cement plaster, with metal mesh reinforcement. The exterior is cement stucco panels with heavy metal reinforcement. My DH says this is a house to stand up to the climate. It does make wireless reception inside a challenge though.

The boards are 3 1/4" on the face, probably 3 1/2" including the tongue.

I initially thought it was heart pine, but the grain just isn't right. There are no knots, anywhere, other than small dark patches where a branch was. Plus, the house was built in 1945, which is a bit late to be putting heart pine down and then covering it immediately with carpet.

The house is in the Pacific Northwest, and I understand Douglas fir was fairly common as a flooring material here.

It sure looks like douglas fir to me. My whole house, flooring and trim, is in doug fir from 1923. I sanded down to 180, waterloxed and then, because it was still splintery and the waterlox was too shiny, finished off with a coat of satin oil-based poly. (Today I might try a product like OSMO instead, but that will have to wait until next time.)

You can see the grain variation in the doorframe and different sections of floor:

That's very satin-y. Looks lovely. So which Waterlox were you using? Original, satin, high-gloss? I want it to finish out somewhere between shiny and satin. The Original says it finishes fairly shiny, and then fades to a 50°  55° gloss level in 4-6 months.

I guess I can do one room and see. I didn't know you could poly over Waterlox... I remember reading that you couldn't, but I can't find that now. I don't see why it would be an issue if the Waterlox was fully cured and had formed a hard, continuous resin layer.

We used Original Waterlox, which is all we could find in our area. Had I been able to wait, I would have tried the satin for a few more coats. Thus, I can't speak to the level of sheen that might ultimately result on a purely Waterloxed floor. The Original has a pretty hard gloss to it.

As is so often the case with DIY, we were utterly under the gun to get the floor done by moving time, so between that and our concern about splinters, we used an oil-based poly (Fabulon) for the final coat. It has held up beautifully -- even in the bathroom -- I contacted the Waterlox folks and they said they couldn't foresee a problem using an oil-based urethane over it (water-based might be a different matter?).

Waterlox (regular and low VOC) does have an extremely penetrating odor, worse than regular poly, just FYI for planning purposes. You won't want to be inside with it for a few days.