Merkelbach was a portrait photographer according to [1]. The Atria page lists the image as in the public domain. Atria is an institute that is a descendant of the International Archives for the Women's Movement. Since starting in 1935, they received many personal archives. It is likely that this photo was never published until public domain. The site hosts other Tendeloo images, and lists those as in Copyright.
This work was never published prior to January 1, 2003, and is currently in the public domain in the United States because it meets one of the following conditions:
its author died before 1954;
the death date of its author is not known, and it was created before 1904;
it is an anonymous work, a pseudonymous work, or a work made for hire, and it was created before 1904.
The above provisions are contained in 17 U.S.C.§ 303. See also this page for more information.