Yes, history is sketchy on the beginning of the Rock. The Barred was first. There's still debate as to which foundation breeds were blended to produce the Barred Plymouth Rock, but Cochin, American Dominque, Black Java, etc were likely blended into the mix. It is claimed that the Whites were sports out the early develpment programs, but who is to say? That too is pretty cloudy historically.
The Columbians, Buff, Silver Penciled, Partridge, etc are all radically different in their composite of different foundational makeup.
What makes a Rock a Rock? It must conform to the agreed upon "standard" for the breed description which, with rather small touches, has remained relatively untouched for 150 years. Pretty good I'd say. The TYPE makes a breed, not it's composition of foundational birds used no matter how diverse. If you can put the gentics together so that the out come in silhouette is a Plymouth Rock, then it got accepted as a Rock.
Apart from a few foundational birds of landrace stock, all breeds are man-made mutts/mixes/blends that finally come to breed true to a descriptive standard. The Plymouth Rock dates to the perioed following the Ware between the States. Some breeds have only a few years of history and are still very much works in art projects in progress.
The composite nature of the many of the birds is still being tweaked and touched to bring the birds back in line with the Plymouth Rock breed standard. Some of the varieites are in desperate shape, others just in poor shape while other varieites are widely held in pretty decent shape. My personal feeling is that the Buff, Partridge and especially the Silver Penciled need the most work, by far. It is very difficult to find good examples.
The White Rock is in the best shape. Just MY opinion.
I just wish the variety held more common appeal. Sadly, it does not. Most folks want pretty lawn art in fancy, smachy shades and colors. Shrug.