Word that combines “indicator” and “metaphor”

I feel like there's a word I can't put my finger on that captures the idea of: an object or entity which can be taken as a case study of a larger phenomenon, a lens through which to view the phenomenon, an embodiment of its principles or contours, the broader phenomenon "writ small," as it were.

The context is: Someone asked the question "When did Stradivarius violins become [such a big deal, the best of their class]?" And someone else pointed out that society does the same thing to non-musical entities like cars, people, institutions (hyper-celebrates one entity to the exclusion of all peers), but I wanted to defend the relevance of the original question to music history by suggesting that the reception history of the Strad (and the violin in general) can be taken as a [missing word] of the reception history of "classical music" (ya know, with scare quotes). That is, in the way the Strad went from a valuable commodity commissioned by nobility, to a 19th-century focal point of the aesthetic elevation of music as an art object, to a pivotal "household name" in the mid-20th-century movement that both democratized classical music and simultaneously charged it with even more class distinction than ever… we can understand the course of the forces shaping the commodity that is "classical music" by following the history of the Strad.

So what I'm looking for is a word (or phrase, but I feel like it's a single word) that combines the idea of "metaphor" (object through which you understand something else) with "indicator" or "representative." I'm also hoping for an explicit connotation of learning a lesson, reaching a conclusion, or achieving new understanding.

Words that have fluttered around my mind and are not quite right (edited to elaborate on words suggested here):

• telltale (in the nautical sense): Has an unneeded implication of forecasting future trends
• bellwether: I don't need the causative implication that X leads Y
• embodiment or incarnation: On the right track, since I'm looking for the notion that "X has the properties of Y, in microcosm," but these have the added meaning that "The whole of Y is contained in X," or "everything true about Y is also true of X," where I'm going for the other way round. (Note, I wound up going with "embodiment" for the moment.)
• typical example: I'm hoping for a word that doesn't make X a peer of Y, but a subset of it. In other words, X isn't a type of Y, but a part of it, or at least a smaller thing through which the larger Y may be understood. E.g. "The Colt Peacemaker is a ___ through which to examine the history of the American West."
• icon, symbol: I'd love to have the word work equally for something un-iconic and un-celebrated. "This obscure painting is a ___ of the entire Impressionist movement."
• prototype: Can have the causative implication that X is the model for Y, that Y derives from X. Also, makes X an instance of Y.
• archetype: ditto
• metaphor: This serves the "we learn about Y by looking at X" model, and really might be as close as I get. But it carries no implication that X and Y are in any way related.

(And, as is so often the case in these questions, I'm open to the possibility that such a word is just a figment of my overheated imagination.)

SAMPLE SENTENCE: The transition from longbow to crossbow is a ____ of the changes in Medieval warfare as a whole. (Invented example, not necessarily good history.)