On Eames fiberglass chair producers : a question about the timeline  

I've got a hole in my understanding of Eames fiberglass chair producers. Indeed, it's OK for Zenith between 50-53 + 1954 for large shockmounts.

However, the first shell I have ever seen from Cincinatti Milacron C* is February 1960. This is from the patent and/or the ink stamp.

Before that, I saw many Summit plastic (twin peaks logo, or double triangle if you wish).

However who produced the others in that period of time 55-60. Are they from Zenith because sometimes you have the Z with 3 stars or from another company when there is nothing. From the edge I guess there are not from Summit (those have a thiner edge) but I'm not sure where they come from.

A also wonder when Summit changes his loggo for the circle S. The first copy I see with that S is 1972 or 1973.

Any comments on this?