Could be almost anything. If you're going replace the thin slat florring anyway, then have at it and remove the stuff; you will then reveal what you have beneath it and be able to decide if it is worth restoring. I have done many restorations and there was only one floor that was DOA. Sounds as if your house has had lots of "remuddling" over the years; it's amazing what a DIYer with poor vision can do to nice architecture.

The Janka hardness test measures the resistance of a sample of wood to denting and wear. It measures the force required to embed an 11.28 mm (0.444 in) steel ball into wood to half the ball's diameter. This method leaves an indentation. A common use of Janka hardness ratings is to determine whether a species is suitable for use as flooring. It should be noted that the test is meant to be used only for "unfinished, open grain flooring" manufactured before the 1990s. With the advent of pre-finished flooring in which hardwood floors are treated with aluminum-oxide based sealers that often double or triple the dent and scratch resistance of the flooring, the Janka Hardness test has been essentially rendered useless in modern-day hardwood flooring, unless the end user is purchasing unfinished or oil coated flooring. From Wikipedia.....

I have an old Victorian in MA, around 140 years old. I finally had to replace the front porch decking last summer. As best as I could tell, we removed the original mahogany decking. If not original, it was at least 30 years old as my neighbor has lived there for 30+ years and has never seen the porch replaced. The frames of the deck were actually attached from inside the house and the vertical porch supports were old wood stumps. It had to be replaced because 2 of those stumps had rotted and one of the 2"x6" (real 2x6) had split. The rear porch of the house still has the old wood that is very sturdy still. The only downer I put on it is the fact the mahogany has greyed severly, but I'm going to be looking to fix that this summer. As far as wear, it has help up nice. (Then again, this house has the original wood gutters still in fine shape). To the question, When I replaced the porch decking, I replaced it with tongue and groove Mahogany, 5/4". So far it has held up well. I'm sure the composite would hold up and require less maintenance, but I (and more importantly my wife) hate the look of composite. The porch is covered, but not enclosed so I'm have yearly maintenance on it. It is our front porch and gets a fair amount of traffic and wind blown weather. The stairs are actually fully exposed and the original wood decking was in excellent shape when removed. The stairs were only replaced due to the lack of any concrete footings so the wood rotted from underneath.

Marti - we have floating engineered hardwood over slab. Can you get anyone to see of you have any old plumbing buried in the slab that is leaking? This is be of the things we did. We had floating engineer wood from Summitt Forest Products. It had a nice handscraped look and none believes me when I say they are engineered. They don't look site finished but do not look like Pergo either. We had it glued near transitions to tile etc. Looks like you are stuck in analysis paralysis. Consider delegating this decision to another person who will just research and get it done and then you can move on to other things. I found that in our remodel, DH and I needed to delegate to one another a few times to take a break and get some distance. Good luck!