Eames Aluminum Group Lounge Chair - Which one to keep?  

I've had a few of these chairs over the years. What I do know is that the "Patent Pending" stamp means it's a first year production model from 1958. Assuming the other chair has the same "contract" base, then you could date it between 1959-mid/late 60's possibly. Around that time they switched to the universal base.

Like you, my first year production model is missing a washer between the bottom of the seat and the top of the base. There is a small gap there that makes the chair just a hair wobbly. I tried a nylon washer (3/4" inside diameter) from the hardware store but I couldn't find anything thin enough to allow me to still fit the retaining clip at the bottom.