Did Eames X-base chairs always have the large shockmounts?  

I have a few shells that have small mounts with original "as-found" x bases.
Both arm shells and side chairs occasionally have small mounts with x bases.
The date is probably around 1954-ish , but dates could be skewed by the dreaded "leftover stock" theory as well. Could even be as late as 55 or 56, if they had some old x bases laying around the factory from way back, as the small mount shells started being produced.
I love finding transitional pieces.
Hope this helps.
EDIT: Hmmm, on further consideration of the photo, it looks from the "waffle pattern" on the shell that it might be a Milacron Plastics shell. You can ask if there are any embossments on the shell near the front edge of the underside. It would have a "C" with a star in it, if it was Milicron, and if so it is possibly on the later side of the dates mentioned above. Either that, or the base may not be original to the shell. Not sure how early Milicron Plastics got started.

Very helpful as always - thanks! I asked the seller, and he said that one of the chairs is marked, and one isn't (it's part of a pair - the second one is also salmon, but has an H-base). I'm assuming that the x-base chair is the one that isn't marked.

You might also ask if the legs are still able to be removed from the base.
It is possible that the screws might be "frozen" in the shock mounts due to rusting. Looking at those rust trails, the chair was possibly stored outside in the upside-down position for a season or two.
Hey but at least that would be proof that the base and shell are an original pair!
Still a cool transitional find in all likelihood.
I estimate that this x-base-on-small-mounts pairing shows up less than once in every 431 shells that I view online. But who's counting...

I had (and recently sold) a pair of chairs with X-bases and small shockmounts. Definitely original.