You cannot steal paternity of the work. Right to paternity is unprescriptible. Nobody can publish Hamlet under his own name claiming that Shakespeare died more than 70 years ago. Nor can anybody publish Small Village, the unknown XVI century tragedy, because the author is unknown
You can certainly print the pictures and state that the photographer is unknown, and you are the printer. You have the copyright of the work as printer.
If you make some profit out of it, and one day the photographer proves his paternity, if the work is still covered by the economic rights, he'll be entitled to a certain share of your profit, which I cannot quantify. If the the work is not still covered by the economic rights, and you did not "steal" its paternity, I certainly presume nothing is due to the photographer.
The fact that you don't know the photographer identity doesn't allow you to hint the work is yours. The fact that the economic rights of the work expired never allows you to hint the work is yours.
The fact that you own, as private property, the negatives doesn't mean you can hint the work is yours. You can own, as private property, a work by Picasso but you cannot hint it's "yours" because it's "yours".
Considering that normally the print is executed by the photographer, in my opinion if you exhibit the work with your name only you are hinting the work is entirely yours and you are in breach of (imprescriptible) paternity rights.
Stating that the photographer is unknown and you are the printer is sufficient. If you become rich because of those prints, be prepared to share the wealth with the photographer's heirs, if they ever trace your work and prove their title.