There are plenty of instances in US folkloric history of collectors publishing a song which had existed in the public domain, publishing it for the first time, and having copyright, at least in practice. That copyright would, it's true, be only on the version the collector published, but if that's the only thing out there in published form, that's what almost anyone is going to be working from, and they'd be bound by the copyright. At least that's my take on it.

Now, Q said:
The comments by Hernandez on US MSS are pertinent.
If you discover a long lost Bach MS score, the form in which it is brought to publication is copyright. Most orchestras or individuals wishing to perform the work would buy the published, copyrighted score.

Seems to me the holder of the Bach score would be in a similar position. He could arrange with an editor, under contract, to place the score in modern form, reserving all rights to the holder. If the published edition is a good job, it will be the standard form of the Bach piece. At least to begin with.

I suppose the holder would have to publish photos of the manuscript and description of the provenance of the manuscript in order to establish that his modern edition is indeed of Bach. And, yes, others would get out their own editions of the Bach piece, working from the photos of the original, but the first edition out would have a running start in the market.

That's my $.02 worth.

Dave Oesterreich