Your question has two aspects, which may not be as related as you think, but I will try to address them both.

First, as to the history of the R. Long & R.R. Smith company: the following information was gathered by drawing on the expertise of the EXLIBRIS-L email discussion group. An index of Publishers' Trade List Annual contains this entry: "Long, Ray, & R.R. Smith, Inc., NY, 1932; Long & Smith, Richard R., Religion Publications. See Harper & Bros., 1934; see Julian Messner, Inc., 1935-1945." This would indicate that the company was transferred to Messner. Subsequently, "Julian Messner Co. became a division of Simon & Schuster in 1977; presumably all of their copyrights became the property of S&S at that time."

However, the work in question appears to have been first self-published as "...in pictures" in 1933: see this WorldCat entry - which lists publication info as: [White Plains, N.Y., Hugo Gellert, ©1933] A search in the library catalogs of the two institutions listed (Yale and NYU) did not find that edition. But this information makes it clear that the copyright was registered to Gellert, not the publishers.

According to this chart of copyright terms, a published work with a copyright notice from 1933 is in the public domain if the copyright was not renewed after 27 years. The University of Pennsylvania' Online Books site has a page on How to find out if a copyright was renewed. One of the resources they refer to is Stanford's Copyright Renewal Database. I did not find Capital in Lithographs in that database, but for real due diligence you should check the paper/microfiche volumes, which are held by large depository libraries.

You may be interested to know that there was a German reprinting of the book in 1981 - see this WorldCat record, but it is only held by two European libraries, so it will be impractical to examine that edition for copyright info.

The last avenue of due diligence I would suggest is based on copyright information for a lithograph in the Library of Congress’ collection, from what appears to be a French version of the same collection, which is credited as "copyright the Estate of Hugo Gellert":
The Working Day, no. 37, ca. 1933. / Hugo Gellert, 1892-1985. / Lithograph. Published in Karl Marx in Pictures (Paris: E. Desjobert, 1933). / LC-USZC4-6586 / © the Estate of Hugo Gellert. (See this page.)
In response to a query, a contact from the Caroline and Erwin Swann Foundation for Caricature and Cartoon said that the right to use a Gellert image on the LOC website was secured from The Mary Ryan Gallery.