Looking for info on Thonet chair  

I don't know anything about it but I do know that Thonet has been in business for a very long time and they've produced many different pieces in a lot of styles over the years. You might try contacting them directly to ask. There's not much on their site about history and old catalogs of inventory but maybe they have a company historian on staff who can help. It can't hurt to try, right?

https://www.thonet.com/contact

Your oak chair is definitely made by Thonet (USA) in the 1900s in the Arts & Crafts "style". The wood plugs in the back indicates a factory made mass produced chair & although it looks Arts & Crafts, it is the opposite of that design movement was all about. It was probably ordered in large quantities for an institution or commercial use & researching that may or may not reveal provenance. Your chair even though made by Thonet Bros (Gebruder Thonet), it does not represent what the company is well known for, the steam bent wood furniture of 1859 & the Bauhaus furniture by Breuer in the 1920s.

Chair models no. 81 (1900) & 511 (1905), see attached image shows the bent wood application in the Vienna Secessionist style even though also mass produced shows the steam bent wood process that Thonet Bros was known for. The tall back chair model 511 was used in a New York City restaurant in 1910 & was included in the Thonet Bros catalog from that period.

Your chair is interesting & you should conserve it as found, maybe sometime later, vintage photographs might show up & might reveal its history or something.

Not sure if the current Thonet USA is still related to the original Thonet.

According to Wikipedia, the original (sort of?) Thonet is still in operation in Europe. The one in the US is an offshoot that broke off for good in 1987. They have some pretty original looking Thonet designs on their site, though--but credited to some young guy. Ok....

I forgot to add that the little tag that says Commonwealth of Pennsylvania, Department of Labor and Industry was (or possibly still is) the owner of that chair so make sure that your ownership is legit or PA state might still want it back like those WPA Art, they are all owned by the US government & a civilian who displays it at home & claims it was acquired legally could be charged with theft or possession of stolen property.

Update: One of my co-worker (from PA) said that the PA label could also just be some sort of stamp of approval (like UL for lamps)...

Thank you all for your input!

I contacted Thonet here in the US and they told me to contact Thonet EU and sent me a link for their identification department, just sent it out a few hours ago so probably no word back until tomorrow. I am very familiar with the bent wood process that Thonet is famous for and that is why I find this model so interesting, as well as the design on the seat back... looks African inspired, quite interesting!

I will pass on any info I get back from Thonet EU.