Is there a single word for "a sub-component of a scientific contribution?"

I doubt that there is any single word that stands out from a list that includes part, section and chapter. Another possibility is division, which may be too mundane a description for the weighty writings you describe.

"Davidson Writer" favours parts

"Five Parts of Argument: Most written arguments that you fashion in your Davidson courses will include five parts: a claim, reasons in support of that claim, evidence your which those reasons rest, assumptions and core values upon which your claims and reasons hold true, and acknowledgment of alternative claims that have been or may be made."

Davidson Writer

chapter = "any of the separate parts into which a book or other piece of text is divided, usually given a number or title"

Cambridge Dictionary

section = "one of the parts that something is divided into"

Cambridge Dictionary

If the work is as important and significant as you suggest, it might be described as a set of five separate theses. Although we often think of a thesis as single body of work, there is no reason why a substantial investigation such as a 40-year monograph could not comprise the discussions of several theses, each of which could stand alone.

thesis = "the main idea, opinion, or theory of a person, group, piece of writing"

Cambridge Dictionary