[Galin]

The Himling text and the early map drawn by JRRT appear to date about 1940.

The external history implies that Himring was the later form of the name for the hill. Christopher Tolkien notes that Himling was the earlier form of Himring in his introduction to Unfinished Tales (about the map), or for example, see the Hill of Himling in The Lay Of Leithian, and commentary: 'Two new elements in the geography appear in this canto: the Hill of Himling (later Himring) rising in the east of the Gorge of Aglon (2994)'

It appears Christopher Tolkien drew the general map published in the First Edition (he made it in 1953), but this detail got left out for some reason. Then the poster-map comes around years later, about which Tolkien added details: 'She [Pauline Baynes] consulted with Tolkien, who sent her a marked photocopy of the general map, as well as additional names to include and advice on a few points of topography and nomenclature...' Hammond And Scull RC

And later we have the detail appearing on the map made for Unfinished Tales of course.