Arne Jacobsen Egg chair - A Guide to Determining Date of Manufacture  

Thin Feet on Early Chairs
Now I'm not sure that any very early chairs had thin feet. One of the examples that lead me to believe they did, turns out to have new feet. See link to Condition Description.
http://www.liveauctioneers.com/item/17662756_early-arne-jacobsen-egg-cha...

year knock-offs started
Does anyone know if "knock-off" versions of the Egg Chair used the "fluted" base.... or are knock-offs a cylinder base only(part that joins swivel piece)?

Don't know
I'm not sure when the flood gates opened on the cheap knock offs, but every one I have seen had a cylindrical two part base. I've never seen a knock off with the cast one piece base with the fluted column.
It has been my experience that the earliest chairs had the thin feet and that the thicker feet were a later modification. You can buy replacement feet in both thicknesses.
I don't put a lot of stock in the placement of the hole for the set screw as a means of dating the chairs. This seems like something that would have been done by hand (drilling and tapping the hole) not something that would have been in the mold.
Far more interesting I think is the difference in the bottom of the cast base where the four legs converge. Some are perfectly smooth, some have a dimple in the center, I think later production have numbers and even have "Fritz Hansen" cast in the mold.