>Codebooks aside, this may (if true!) be the earliest recorded instance of >data-compression.
Not unless the VMS is older than we think. There is a squiggle in Chinese which means "repeat the previous character". There is one in Japanese which means "repeat the previous syllable" and another which means "repeat the previous word". And the modern "simplified characters" are nothing but an adaptation of the cursive script (aka caoshu, or ts'ao-shu). Come to think of it, Ancient Egyptian hieratic and demotic are likewise "data-compressed" hieroglyphs. And this brings to my mind the Tironian notes, which were the granddaddy of shorthand. Closer to us, you often see a "2" in Indonesian and in Malay, which just means "repeat the previous word." It comes in handy when you have to write "mahasiswa-mahasiswa, mahasiswi-mahasiswi" (mahasiswa is a university student, male, the other one is ditto, female; it's borrowed from Sanskrit, of course).