As expected, this old chair has seen a lot of heavy use and there is much evidence of it.
These are plugs for the screws that hold the two pieces forming the backrest of the chair.
Luckily, all of the apparent damage can be fixed and is not structural in nature - always a good thing when dealing with chairs since it bears a lot of force on the joinery and construction by the user.
The seat is obviously a previous repair. The 1/4" plywood is correct in nature but the execution was poor, primarily because the blank was not cut small enough to fit inside of the seat channel or rabbet. Also, I believe the the original seat blank was glued into the rabbet and not nailed as you see here. Sadly, not having the original seat blank makes it harder to verify any maker's marks such as a label or stamp - usually found on the underside of many chairs. The seat blank will be replaced and I may be open to fitting this chair with a removable upholstered seat cushion that would make sitting more comfortable for long-term use.
I really appreciate the design and shape of this chair. It flows beautifully, even when viewed from the bottom.
Some say that Thonet made the idea of assembly furniture (now dominated by Ikea) popular as early as the late 1800s-early 1900s. This chair is primarily held together with wood screws, two metal brackets, and very little wood glue. 
More evidence of why this replacement seat blank was done poorly and incorrect.
If you look closely (on the left), you can see that the front leg is separating from the seat ring.
The legs are in poor shape, each in need of some repairs or reinforcements, some more than others. How I approach each repair will be discussed in future posts, stay tuned.
Lastly, the current finish on this chair was stained over by evidence of one of the broken legs with missing wood having stain on the exposed wood. This makes removing the finish an easier choice since the original finish and patina is already covered up.
In my next post, we'll begin disassembling the chair so we can start the refurbishing process...
