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.)

This obscure painting is the epitome of the entire Impressionist movement.

I believe this word captures the concepts of symbol, perfect example, and distillation/microcosm/encapsulation.

epitome (n.)

The most perfect type or example

A visible representation of something abstract (as a quality) m-w

Transferred. Something that forms a condensed record or representation ‘in miniature’. OED

The embodiment or encapsulation of a class of items. Wiktionary

Is a Stradivarius violin the epitome of a particular musical craft or is it a work of art? L. Haywood et al.; Understanding Leisure

Yet the question arises whether the scale model ... does not embody, in all cases, the epitome of the work of art. Didier Maleuvre; Museum Memories

As I will show, railway modelling is the epitome of a constrained utopia, a vehicle through which subjective desire and technical skill are manifest, which is nonetheless beset on all sides by various structuring mechanisms. Stephen Knott; Amateur Craft

The lowly sign of the cross is the epitome and manifestation of the whole cosmic process, for all things in nature are included in the drama of the world's redemption on the Cross, and in the four dimensions of the Cross the ancient Christian...saw the four dimensions of the cosmos suggested as in a mystical symbol. The Cross is the "recapitulation" of the work of Creation, it is the epitome, the simple sign, the sensuous symbol of something vast and unknown—in short, it is a mystery. Joseph Campbell; The Mysteries

Among the important 'cosmological symbols' is that of the 'Tree'. According to the Sufis, the epitome of knowledge is expressed through the 'Tree of Knowledge', which has its roots in the phenomenal world but is also connected with the World of Archetypes... Laleh refers to Ibn Arabi's description of this symbol as the epitome of knowledge: S. Fahim; Doris Lessing and Sufi Equilibrium

It seems that the word you are trying to identify should be "archetype".

(OALD) archetype noun BrE/ˈɑːkitaɪp/ AmE /ˈɑːrkitaɪp/ (formal)
​the most typical or perfect example of a particular kind of person or thing
♦ She is the archetype of an American movie star.

(ref.) This raises questions: does archetype semantics in its traditional sense correspond to the concept of “an image” if we begin considering an image specifically in terms of “a copy”, “a duplicate?”

(ref. 2) It may be noticed that the notion of “great public images” taken over as models for narrative shape tends to overlap considerably with the concept of archetype introduced in preceding chapters.

Addition prompted by comment from user Andy Bonner

The word template used figuratively carries the idea of "representative" and at the same time, although not as vividly, that of "most perfect instance" or "best in the class"; however, there is no metaphorical concept involved: the template is not something else. Incidentally, let's mention that the idea of the coincidence for a given term, of it being a metaphor for a concept and of its referent being a representative of that concept is contradictory.

(ref.) Al Gamper explains how his family life became a template—both a gauge and a pattern—for developing the habit of mind of conviction about corporate equity.

(ref.) the Ten Commandments—ten principles that became a template for all of Jewish life for millennia to come.

(ref.) The ideal is usually identified with the emperor Asoka , who became a template for Southeast Asian monarchy .

It seems that word would fill the gap in "The Colt Peacemaker is a ___ of 20th-century firearms", even if not as ideally as some might wish.