I don't know if these are Thonet chairs, but they are nearly identical to the Thonet S33 and Desta ST11 chairs designed by Mart Stam. The S33 has a chrome frame with a moulded plywood back and seat, while mine have chrome frames with an anatomically-contoured steel seat and steel back that curve along the same lines as the S33.
My guess is that these were manufactured circa 1930.
The black finish on the seats and backs of my chairs seems to be a hard, baked-on plastic or incredibly-durable lacquer paint. It is chipped in some places, and slightly cracked in some others due to age, but the major blemishes have been touched up, and all of the surfaces can be sat upon without concern. There are also countless little dots of minor surface corrosion on the chromed frames, but most can be scraped off with a fingernail, and therefore, a good polishing by hand should make them even prettier than than they are at present.
Here are the closest thing that I've found to them.
See: https://www.1stdibs.com/furniture/seating/dining-room-chairs/mid-century-brushed-tubular-steel-contoured-cantilever-chairs/id-f_26798922/?
As for their age, I don't know how old they are, but all of the screws are made for a flat head screwdriver, and so that, combined with the fact that I've found it impossible to locate more like them, indicates that they are definitely the real deal.
You can get a good look at the S33 here, although please note that this person has them listed as a set of S43 chairs, which I believe is incorrect, as catalogs cite the S43 as the armchair model:
See: https://www.pamono.com/s43-tubular-chairs-by-mart-stam-for-thonet-1930s-set-of-4
--- midcentury mid century modern cesca lorenz bauhaus bau haus tubular ---