I'm attempting to demonstrate that a film registered in 1947 did not have its copyright renewed by the registrant in its 28th year, and thus would have fallen out of copyright and become public domain (because it was published between 1922 and 1964 before the law with respect to renewal changed). The film itself is already present in several public domain archives, so there is good reason to believe this is the case; however, one needs to be certain about these things before publishing a work that incorporates the original.
I can pay a firm to do a search of registrations and renewals based on the data present at the Library of Congress (the data is not present in the LOC's online search due to its age), or I can pay them a lot more (4x) to do a more exhaustive search for assignment of the copyright to other entities as well.
My question is this: if the production company had assigned or transferred their copyright to another entity prior to the 28th year, and that other entity had renewed the copyright themselves, would we still expect that renewal to be reflected in a search of registrations and renewals for the copyrighted work?
Alternatively, is there a better approach to demonstrate that a work is now public domain?
(If so, then presumably the less-exhaustive but much-less-expensive search should be adequate to show that the film did not have its copyright renewed in 1975 and therefore became public domain.)