Your interpretation seems correct. I believe "to which are assimilated" indicates that "works by a process analogous to cinematography" are grouped together with "cinematographic works".
Therefore the sentence would be easier to understand if stated as :
"Cinematographic works which include works by a process analogous to cinematography"
or
"Cinematographic works, including works by a process analogous to cinematography"
The original language of the treaty - "cinematographic works and works produced by a process analogous to cinematography" - has an almost identical meaning. The difference is that the new language suggests that "works produced by a process analogous to cinematography" is a kind of "cinematographic works", while the original language suggests "cinematographic works" and "works produced by a process analogous to cinematography" are two separate terms.
I am not very sure whether they just changed its wording to clarify the meaning or intended to change the meaning with the amendment in 1967
While the overall meaning seems to remain unchanged (since whatever it was that applies to both "cinematographic works" and "works produced by a process analogous to cinematography" still applies to both), I find the new language harder to understand, so I wouldn't call that a clarification. But legal documents are often not meant to be clear.
