OK, I found one of my sources.
The striated plate is the later plate according to "A Guide to buying, Upgrading and cleaning a Featherweight" by M. Wood and T. Wood, circa 2007.
I should mention too that the plates changed right around 1950. So there was a period of flux there where a centennial could have the striated or the "Art Deco", "Celtic Knot" Floral" whatever you want to call it, but all the way into 1951? Less likely.