Malafouris and Renfrew discuss the Mycenaean Linear B clay tablet as a prototypical exogram, hence as a kind of extended memory. They, then, proceed to note differences between it and engrams. And even note that we should be careful not to overlook the differences between engrams and exograms. This, however, is what Adams and Aizawa and Rupert have been emphasizing. The differences are significant enough that, while we might take a vague similarity between them as motivating calling them "memory" (as happens with human memory and computer memory, for example), the way of the scientific future should be not to just lump these exograms and engrams together in a generic "memory science".
Malafouris, L., & Renfrew, C. (Eds.). (2010). The cognitive life of things: Recasting the boundaries of the mind. Cambridge: McDonald Institute Monographs.