Arne Jacobsen Series 7 on casters from the 1950's  

The problem
is at the flex point.

This is a very common...
This is a very common problem with these chairs, perhaps the most common problem. I have no experience with this specific issue, however, I am sure that you will find many people who have.

Series 7
An inherent problem with this and other Jacobsen plywood chairs... I would not even let anyone sit on a chair that isn't cracked. I have one for display.

I am bound
I am taking a deontological stance and am admiting my duty in adhering to a promise that I would never sell or get rid of the chair. This was part of the deal so it may be the "one" that remains objectified.
However, I wonder if Fritz would just sell me another shell or two. Seriously, there has to be someone out there who has a bummed base and needs to scrap the shell.
Are these rare in stock with the original base? I have seen the ones with the new caster bases and ones with a naugahyde coating but never this exact one. Perhaps this has a specified name and that is why when I google image series 7 chair on casters, this example remains hidden.

Model 3217
If you search for something like:
vintage jacobsen "series 7" 3217
you'll find pages like this one that show your chair:
http://www.deconet.com/product/13854/

Fragility
I know the Series 7 and Ant chairs have a reputation for fragility, and I've seen a number of them for sale with softness -- or sometimes even visible cracks -- at the flex point... But is it possible that the designs were subtly changed at some point to strengthen that area, and that the newer ones are more sturdy?
I ask because my 3217s SEEM thicker in that area than some others I've seen... But the last time I saw another one up close was before I got my chairs, so my recollection could easily be wrong.

I did and none like the one...
I did and none like the one that we have appears. Only like the example above and newer models. Yet, I ask are these not common or did someone in the past remove the vinyl thus exposing the wood? It is plausible that the architect or someone before him just changed out a regular series 7 arm chair and placed the old caster base on it.
Fastfwd that might be the case since the date on the bottom states late 50's. I will check and post.

Yours
Looks like teak, which was one of the most common veneers for this chair. Your chair is most likely in its original iteration, in my opinion.

Lenox
Sorry, I misunderstood what you were looking for; I didn't realize that you wanted examples of other 3217s EXACTLY like yours, with no upholstery.
I suspect that most people who bought a new 3217 in those days wanted upholstery on their expensive new office chairs, so yours might be rare. These days, now that the Series Seven is such a design icon, the clean look of the unupholstered chairs is more popular... Although I must say that I find them mildly uncomfortable: I slide forward on the slippery bare wood and can't really sit straight for very long. Hmm, maybe if I wore vinyl hotpants...

Buy a vintage seat
I've had both versions you have there. Four-legged arm chair and chair with arm and caster base.
It's sad but true that the series 7 chairs will commonly break at the waist like that, it should not be difficult to find a couple vintage chairs that are still in good condition for not a lot of money.
Buy a couple vintage series 7 side chairs, put the seats on your bases and you are back in business.

Lenox, a helpful tip for you....
for a more focused search, instead of google use architonic's "Design Collector" auction house archives.
Here are some other examples of your chair, and there are many more:
http://www.architonic.com/dcobj/search/arne%20jacobsen/3/1

Finally
Thank you for the advice. I usually get emails from folks outside of here who ask me various questions related to mid century furniture design. Eventhough I am in the novice stage I seek to be informative so that I may rightly help another collecter. Thank you greatly for the source, In Solidarity

Wheel dimensions?
Thank you so much for posting these images (nearly 3 years ago). I was having difficulty finding this design with 4 metal casters, vs. 5 black plastic ones. We have one of these but it's missing all 4 wheels. After much searching, it seems remaking them is our only option. Is there any way you could measure them (diameter, thickness) and post the dimensions? I would be so grateful. I'm hoping to surprise my boyfriend with them for Christmas. Thank you!!!