Accounts of the early history of particle physics state that, between the late 1940s and early 1950s, experimenters discovered new particles which theorists then classified into groups thanks to the discovery of what is today seen as special “particle properties,” like “baryon number” or “strangeness.” This narrative fits the ideal division of labor between experimenters and theorists, with the results of the former providing the “factual” basis for the latter's work. However, historical sources do not support such a clear-cut picture: the properties and symmetries today defining particles were not derived by theorists from “facts” provided by experimenters, but rather emerged from a protracted, multilayered interplay of experimental and theoretical activities. Approaching this historical constellation by focusing on the representations used by scientists to express their results is a heuristically fruitful strategy to reconstruct the processes in which concepts of new “particle properties” emerged. This paper shall discuss the most important of such forms, and the goals and methods of the scientists employing them.