There is no "standard" slate. Just about every production has it's own slate, custom made to the needs of the production. You can get slates (or clapperboards) from a number of sources. In the 50s and early 60s you will find that they were still using the black slates with white (chalk) writing, the white slates didn't really come into common use until the 80s.
...and no, that's not experience talking about the 60s, I was but a child then.