Aalto stool question  

I don't have any ideas, but a question,
what is the upholstery: fabric, vinyl, leather? I can't tell in the photo.

The upholstery was a tan /...
The upholstery was a tan / off-white microfiber with a myriad of interesting stains. It was removed immediately as it was disgusting and obviously not original. It has not yet been replaced; I'm hoping to get an idea of original upholstery options before deciding what to use.

Hi
I could be wrong, but I thought this type of stamp was indicative of early (earliest?) production.

Notice that the
legs have been repositioned. The original locations are indicated in pencil -- as are the later ones, including a number (4). One of the original screws seems to have broken off in the plywood (lower left) -- hard to know how that happened. Outlines are sometimes made for relocation after removal -- as for refinishing.
The center hole might have been used to spin the plywood disc around a stationary router cutter -- I've made many a disc that way.

Here are a few more photos...
Here are a few more photos for reference, showing the entire bottom as well as the ring that slides off.
SDR-
I presumed the hole was to let air escape when the upholstery was compressed. Your explanation seems more likely though.

.
Could you show how the ring fastens on? Is it ordinarily removable or have you pulled little nails out or something?
Oh ok so its just a continous ring with a pressure fit?

Microfiber
is extremely cleanable. Use your time machine to go back and stop yourself from prematurely taking it apart so you can simply clean it up and be able to use it in the meantime, while you consider alternate fabrics.

Heath-
Yes, it's just a...
Heath-
Yes, it's just a pressure fit. It's very snug at the "bottom" (closer to the floor) and has a decent gap at the "top".
ObjectWorship-
I have a funny story about a microfiber couch and an entire bowl of minestrone, for another time. You're probably right, it was likely salvageable, but I tossed it. Partly out of laziness and partly to force myself to find an alternative.
One thing I regret is that in removing the upholstery I pulled out a roll of fabric around the edge (installed to soften the edge, I presume). I wasn't too worried at the time but I'm now wondering if that's a standard upholsterer's trick or if I'll end up trying to explain it to someone and kicking myself for being impulsive.

.
Thanks for the answer, for the very small amount of fabric or leather that would require get the best, you'll probably be able to beg a remnant piece, if there is enough of a gap some nice saddlers leather (the thin stuff for panniers and things) would look good.
Microfibre always looks dirty to me and feels a bit unpleasant.

Heath-
We're thinking along...
Heath-
We're thinking along the same lines. I have a few other pieces to be upholstered and may use scraps; if not it will likely end up with leather. I don't plan to refinish the wood, rather I'll leave it beat up and aged, so I like the idea of a natural (untreated) leather that will show age, but I'm not sure about the light brown leather against the maple.