The label God A, still used for Ah Puch in modern scholarship, comes not from any ancient Maya source but from a nineteenth century German scholar, Paul Schellhas, who in 1897 devised a simple alphabetical system to identify the recurring deities pictured in the surviving Maya codices at a time when almost none of the accompanying hieroglyphic captions could yet be read.
Schellhas assigned a letter, A, B, C and onward, to each distinct figure he could isolate by its consistent visual attributes, deliberately choosing a neutral, provisional labeling system precisely because the deities' actual ancient names were unknown to him, intending scholars to replace the letters with real names as decipherment progressed. More than a century of subsequent work on Maya hieroglyphic writing has succeeded in reading actual ancient names for a number of Schellhas's lettered gods, but God A has proven to be one of the more resistant cases: while colonial Yucatec sources supply the name Yum Cimil and the related term Ah Puch, and further south the Lacandon Maya, a people who maintained aspects of pre-conquest religious practice into the modern era, use the name Kisin for a broadly comparable death deity, epigraphers have not achieved full consensus on how, or whether, these various colonial and ethnographic names map cleanly onto a single consistent Classic-period hieroglyphic name for the god pictured in the codices.
Ah Puch's continued identification by the old Schellhas letter, even after so much progress elsewhere in Maya decipherment, is a useful reminder of how much nineteenth century scholarly convention still structures the modern study of Maya religion, more than a hundred years after its original, deliberately temporary purpose.