I have never seen subfloor that narrow, or run that close together in an old house. It has always been 1X6 or 1X8 with a 1/2" to 3/4" gap between boards. In some cases it has been 1X6 T&G loosely laid together. Also in many cases laid diagonally.
In my opinion, this was intended to be the finish floor that shrank excessively and perhaps face nailed at a later date to reduce/eliminate squeaks. Or the original installer chose to face nail. All the gaps are very uniform, is the reason I attest that to shrinkage. If it was laid as a subfloor the installer would not have taken the time to get it all uniformly tight, since it would have been covered up anyway.
What to do with it...Did you like the way it looked before, but just wanted to freshen it up? stain it, seal it and finish it the way you like.
Do confirm one thing please. The flooring ideally should be tongue and groove (T&G), if you can send a thin sliver of metal between the gaps here and there to see how far the sliver goes into the gap. It should go no more than 3/8" if the flooring is 1X material. I have seen 5/4 material on one floor, so it possibly could be slightly more. Under no circumstances it should go 3/4" into the gap.
Another note, in old houses, bedrooms are usually on the second floor, where a much cheaper (soft) wood is commonly used. Also there is in many cases, no subfloor even under the finish floor at all.