If the planks are under the plywood, they may be the original sub-flooring; and if the planks run diagonally across the floor joists it's most likely sub-flooring. You are going to have to do some investigation to find out for sure. You are best off not using sub-flooring for the finished floor.

Nicole Curtis on HGTV's Rehab Addict had the same scenario and used planks as I recall. She used it. She also goes up to the unfinished attics to see if she can grab wood up there to patch! Get a floor refinisher who has experience with renovating old homes or if you are going to pull in a contractor at some point, begin to establish a relationship now and get his/ her advice. There is a also a pro section on Houzz you can post questions to! Lucky you. So glad you are debating this instead of heading to a box store for fake floor! Keep us posted on your journey!

If there are heating vents in the floor, get a flashlight, get down on your hands and knees, and pull out a vent. That's the easiest way to see all the layers of flooding you're dealing with. And, if you have a basement, look up at the exposed wooden ceiling down there if possible - if it matches the floor above, then your floor is your subfloor. We lived with a subfloor as a floor for 10 years in our 150 yo farmhouse, and I wouldn't recommend it. Dirt and pet hair got clogged in the cracks, and light and draughts and dust came up through cracks from the basement. Early floors weren't generally tongue-and-groove, and it was pretty naaasty. Ours was 2" maple boards, and they had been urethaned at some point as if they were a quality finished floor and not the sub floor - cheap quick fix from the previous seller. ;)

We are renovating a home estimated 1875-1885. We First we pulled back carpet and found linoleum, then black tar paper, then we found Maple planks that sat directly onto the original Hand hewn America Chestnut beams. This was quite common in earlier days. they did not know about dry rot. Keeping the beams, but floor joists and Maple had to go due to dry rot. We are finding the home to have a world of history hidden within. I sure wanted to refinish that original hardwood flooring.