Eames Aluminum Group Production History  

Don't know of any site, but I have done a little research on these, and here's what I know. There are gaps in my knowledge, and some of the dates are approximate, but it'll get you started.
The first Eames Aluminum Group chairs (1958) have the same flat four-legged base as yours, and that arrangement is known as the "contract base." The first year's production or so has PATENT PENDING instead of a patent number cast into the crossbeam. Early on, the line was offered in a mesh Saran upholstery, intended for outdoor use, in addition to vinyl and fabric, and in fact was called the "Indoor Outdoor Group" for a few months before "Aluminum Group" became the name.
Some of the lounge chairs from the early years have black painted arms, and others have clear-coated aluminum arms. The black ones disappeared somewhere along the way, but I couldn't tell you when. 
In the mid-1960s, the base was switched to a version with four taller legs that arc downward at their ends. This was called the "Universal base," and was available on the Eames fiberglass-chair lines as well. I'm not sure whether the contract base continued to be available after that.
In 1974 or '75, the whole production process was revamped, and I believe many of the chair's parts were reworked slightly in dimension and details. Among other things, the back crossbeam, formerly held in place with Allen bolts, was attached with pins. I think the mountings for the arms changed a little, too. Earlier fabric slings will not fit the later chairs, and vice versa. 
Somewhere in there--not sure whether it was '74/'75 or later--the tilt-recline mechanism on the chair was re-engineered. It was formerly built around a rubber-and-steel device called a Flo-Tilt cylinder, which tends to give out with age; later examples have a steel torsion bar mounted in a frame across the bottom of the chair, which twists slightly as one leans back. The new version is more durable but (arguably) spoils the line of the chair somewhat.
At some point in the 1980s (I think; not sure of the date), the four-point base was replaced with a five-point version. Some people think the four-point version is more attractive, but it is more prone to tipping when one leans back in the chair.
Recent changes--in the past ten years or so--have included optional real leather in addition to vinyl and fabric, an outdoor-upholstery mesh option that looks quite a bit like the original Saran upholstery. and the reintroduction of the four-point base as an option on the lounge chairs.
I have one like yours and love it. Best and most comfortable reading chair I've ever sat in. I found it pretty cheap, too.