Documents from archives and Copyright

[history1]

[...] That means it depends on whether (Nazi) German, Soviet or post-war Polish laws were in effect and what they said about ownership of found material.

Why should there be any Soviet law in effect of post WWII Poland?
Even if he found it on the street it wasn´t right to keep it as it´s clearly the possession of another person. In WWII Germany people has been shot for looting in other buildings.

Most likely scenario, as I see it: If it is regarded as "war booty" or the Pole made an effort but failed to locate the BDM girl at the time, the museum will be the legal owner of the album. The BDM girl, or her relatives, will still be the copyright owners. As I understand it, thanks to the EU, they will hold the copyright until 70 years after the BDM girl dies/died (provided it is not further extended until that time has passed).

The Pole who "found" the album didn´t make any effort to find the legal owner of the album, I know this firsthand from a guy of the museum. Nor did the museum itself.
The BDM girls is the only copyright holder, unless she passed it over to her heir as inherited property, and this person to the next successors and so on. Referring to German and Austrian copyright law it´s not possible to sell/present the copyright to other persons/institutions which are purchasing copyright protected artwork/photos from anyone. Fact is that if the BDM girls is the legal copyright owner ( = she shot the photos presented in the album) and she handed the copyright over to her heirs, what we don´t know, it´s clearly a copyright violation.

You'd need a host of international copyright lawyers to get that kind of mess sorted out. I'm certainly no expert, but I know what a hassle it can be when someone has donated photos taken by himself to a local museum without also signing over the copyright. If just one of the heirs is litigious, reproducing such a photo could create a lot of legal and financial problems, regardless of the intentions of the donor.

You/one can only sell the right of use, exclusively if you want, but not the copyright.
This matter reminds me so much on the claim for the return of former jewish possession.

[StefanSiverud]

Why should there be any Soviet law in effect of post WWII Poland?

Not post-war, but during the (quite brief) time between the Soviet conquest and the instalment of Polish government. I would assume Soviet laws would be in effect, as it was the USSR that held the ground until control was transferred.

Even if he found it on the street it wasn´t right to keep it as it´s clearly the possession of another person. In WWII Germany people has been shot for looting in other buildings.

As I said, it might well be so. It depends on the laws in effect at the time, however, so you'd have to figure out when the theft/acquisition occurred and what laws were in effect at the time. After that, you'd need lawyers.

The Pole who "found" the album didn´t make any effort to find the legal owner of the album, I know this firsthand from a guy of the museum. Nor did the museum itself.

If that is so, the above scenario is not relevant, and it would probably be considered theft (again depending on the laws in effect at the time). Which in turn would mean the museum might be forced to return the property stolen, depending on the laws of Poland and the EU.

The BDM girls is the only copyright holder, unless she passed it over to her heir as inherited property, and this person to the next successors and so on. Referring to German and Austrian copyright law it´s not possible to sell/present the copyright to other persons/institutions which are purchasing copyright protected artwork/photos from anyone. Fact is that if the BDM girls is the legal copyright owner ( = she shot the photos presented in the album) and she handed the copyright over to her heirs, what we don´t know, it´s clearly a copyright violation.

That's pretty much what I said. If the museum is making copies of the photos, it's certainly a copyright violation in the scenario you have described.

You/one can only sell the right of use, exclusively if you want, but not the copyright.
This matter reminds me so much on the claim for the return of former jewish possession.

Regarding copyright, that depends on the country and the time. Regarding the likeness to the cases of plundered Jewish possessions, of course it is.