Bertoia pad refoam?  

Yes
I do have some left.
I have to admit I'm not crazy with how they look - a bit bulky especially on my kid's chairs. I'm actually thinking of cutting mine in half. Also, the felt is pretty thick so the chairs stand off the floor a bit. The felt can just be pulled off though. I think a thin piece of cork on the bottom would be a nice replacement.

Math check
Your calculation's a bit off, SX-70. To the nearest millimeter:
7/16"=11mm
9/16= 14mm
You're going to be about 3mm too small with those Ebay glides.

You're right.
I don't...
You're right.
I don't have the chairs right in front of me but they ended up measuring closer to 5/8" I believe. Anyway, the glides fit - actually wish there were tighter.
I assume they increased the gauge of steel for the current production.

Current production
Uses a different style of glide.

Indeed.
Glides are differe...
Indeed.
Glides are different now.
I just mean the diameter currently listed on the Knoll spec sheet is 14mm.
I think you had questioned above whether the diameter may be different for the vintage chairs or, more specifically, for the vintage child's chair. It seems that is the case.

child chair replacement pads
bertoiapad sells them for $59 each. You have to emila them and get a special order.

Quality control?
I have not seen any of the pads from bertoiapads in person so I can't comment of the quality. But if I were trying to generate sales, I'd use images of pads that look better than some of the photos on their site.

Bertoia Glides...
Don't buy those glides from ebay. They suck. I got my first set and they never held down right. They kept flipping to the side. I then contacted the seller and said he had fixed the issue and sent me another set at no cost. The new set appeared to have a lower felt pad. They worked better but then I don't sit too often on that chair. So I had some friends over and the first guy that sat on it broke the damn thing which caused a big scratch on my floor. And he wasn't big, he just sat on it like a normal person would. I'm usually carefull with my furniture so perhaps I sat too lightly testing it?
They are the same as the ones that SX-70 put a link to.
So, if you are worried about your floor. This is not a good call. I don't know where to find the real ones for a vintage chair. If someone does please let me know.
So I've been thinking about getting those triangular pencil rubber grips. They come in transparent, black or funky colors haha. Probably slice one side to slip it in and perhaps they'll work better. At least they wont damage the floor. Has anyone tried this?
http://www.adoremusbooks.com/pencilgrip-triangle-1.aspx

yep - not awesome.
I agree - the felt pad is too tall and they roll over. I've actually taken the felt off for now. Perhaps I'll apply a thinner felt pad or cork at some point.
Overall though, agree....thumbs down on those glides.

Vintage
Honestly, I don't know if the vintage glides would be much better. As the plastic gets older and wears down, they will slide around as well. Also, they don't have any felt on the bottom so they probably not as good as felt at preventing scratches either.

OMG
Free advertising!
http://www.nytimes.com/slideshow/2010/07/14/garden/20100715-location-sid...

Double Perimeter Wire - Bertoia
Hey Woodywood is that a Bertoia repro with that double perimeter wire?
Looks too new to be the real thing!

Haha
Yeah, I was referring to pads, but just noticed that the chairs themselves are indeed bad knock-offs. Just look at the shape of those things!! Even DWR would not sell them in their "heyday".

hmmm
woodywood, are you affiliated with bertoia pads/dot/com?

No
I just noticed the mention in the slideshow and thought it was pertinent to this thread, which is why I posted "free advertising".