Hard pine is reasonable for the species. You will not know if sanding will take out the water staining until you try it. Often floors that have been covered need some repair work - boards replaced. I have seen both pure tung oil and waterlox used succesfully on hard pine.

It looks like longleaf pine.
My 1906 kitchen floor is similar. I had to patch it by taking up 1/3 of it and relaying it to fill in a former stair location, and remove water damage. Fortunately I had stashed away some very similar material from a previous jobsite.
I refinished it with 4 coats of Waterlox.

It's heart pine or fir. I think fir. It will look great when finished.

Could also be yellow pine. In any case it was put down as a subfloor base for linoleum etc. The water damage will probably not come out with sanding. Very often there is permanent discoloration where two pieces of the top covering came together caused by moisture seepage over the years.

Nice picture, love Waterlox myself!