I've been reading manuals for BASIC implementations and the earliest reference I can find to PEEK, POKE, and USR is in the Altair BASIC 3.2 manual, published in 1975. This morning I came across this article, in the March 31, 1976 issue of the Homebrew Computing Newsletter: "Tiny BASIC is a proper subset of Dartmouth BASIC... For the 6800, this language has been augmented to include the functions RND, USR, and PEEK and POKE, giving the user access to all his system components in the 6800 from the BASIC program." Tom Pittman writes as if he expects his readers to know what these are; now, the Homebrew club did have an early incident of software piracy, with 50 paper tape copies of Altair BASIC distributed, so he might have assumed they knew it from Altair BASIC, but not everyone had an 8080, so that seems a stretch.
What earlier usage, if any, is there of these functions in an implementation of BASIC or another language? Or were they a Microsoft invention?