Unmarked Eames Shell Chairs with paint, intriguing?!?  

Early shells
Weren't embossed.

1958?
Pre 1958, correct? These just feel a little newer, they don't have much of a thread pattern and I thought the lemon yellow was a newer color. Does the painted back/cover help date them as well?

Lemon Yellow
Was one of the early shell colors. I believe you are correct that shells were not embossed with Herman Miller until the late 1950s. The shock mounts with holes are also indicative of early production. Many of the early covered shells had factory-painted backs (presumably to be more neutral).

no holes!
actually there are no extra holes, the seller said that the covers were taped down (not factory I'm guessing) I'm so excited to have a full matched set and just want to know every detail!
Any thoughts on removing the grey? It makes them look a little dirty, I did this with a lemon armchair awhile back and it look so 'light' and pretty, but sanding took hours and I don't want to risk damaging them.

Holes
I'm referring to the holes on the shock mounts. Yes, tape was used on the early shells.

I would assume many early production did not have the emboss..
Early upholstered chairs ( started around 1953 ) were covered with a single cover, with foam only in certain areas..with a wire frame to hold them on with tension. These tended to fall off over time. Not until later did they sew them on. The backs were painted to keep the color uniform. These early upholstered chairs were usually B grades considered defective.
So many painted backed shells are just remainders of the coverless upholstered production.