eames fiberglass chair bolt patterns  

Arm chair
Only has one bolt pattern which is known as the narrow mount.
There are some seller(s) who sell wide mount rocker bases (made for the stacking side shells), but I have not seen many listed lately. It is generally stated in the listing.

Also
The narrow mount base for the side chair, and the only base for the armchair should be 7.5" in the front, 4.5" on the back.
Also - you have two types of screws used on the Eames fiberglass chairs- 1/4" 20 thread and 1/4" 28 thread. The former is more common to get nowadays. If you've got the latter you can sometimes find replacement screws on ebay, or you can sub a 6mm metric screw - but it wiggles a bit and requires a lock-washer.

You have to search Ebay for the screws?
Really?
You should be able to find appropriate 1/4"-28 screws from any good fastener vendor. The link below shows 95 of them from Fastenal [wow, second time today I've recommended them]; are any of them right for the Eames shells?
http://www.fastenal.com/web/products.ex?N=999600059+1199944539+119979833...

Fine thread
Screw size for most early shells is 1/4"-28, 1/2" long. Yes, you can get them from most hardware vendors. Big boxes generally do not carry them though (only the 20 thread).
Edit: People sell the original hardware on eBay, as well as replacements (obviously, overpriced).

thanks...
...for the bolt pattern info
1/4x28 should be no problem.
Some back story on my inquiry: I may be able to purchase a 4 seat set of Eames fiberglass arm chairs mounted on a single base for airport seating. I'd like to switch out the seats and replace with individual bases. Keep 2 and sell 2. Maybe break even. My only reservation is that the seats are have the thin vinyl upholstered covering. I'm worried these may not have as much resale value as un-upholstered. Any opinions?
Has anyone tried to peel off the upholstry with any success?

Condition
If the upholstery is in good condition (e.g. no holes, lifting, etc.) and a nice color, you may get a decent price but probably not as much as regular fiberglass in good condition.
My recommendation is to not mess with stripping the upholstery.