The program makes a small historical error in the scene where Peggy first sees her typewriter, an IBM Selectric II. The IBM Selectric Typewriter featuring a ball-element, that made the type bar and moving platen unnecessary was first released in 1961. The first Selectric II, which Joan assures Peggy is so easy a woman can use it, was not released until 1971. Later in the episode, Don comments about the availability of a "magic machine" that makes identical copies of documents. Although Xerox had released its first photocopier in late 1959, it probably would not have been widely available and probably not well known in March, 1960, when this episode is set.
The bolded part needs to be taken out. When Don mentions the "magic machine", the writers were being ironic because he says, "It's not like there's some magic machine that makes identical copies of things." So, clearly, Don has no idea that Xerox exists yet, and there's no time period goof that needs mentioning here.