Marylebone Antiques has dated this piece to the 1880s.
They must have been made by the same chair maker. I think that someone originally ordered the eight & then added another four a bit after, or there were only eight & four available at the time. You can see they are very similar, with very slight differences in some of the carvings, mainly on the middle of the backs. These chairs are top quality, very impressive & have many intricate features. They are structurally sound & in excellent condition. The drop in seats have just been re-upholstered in neutral coloured fabric. Each seat is numbered underneath, which is another sign of quality as each drop in was made for a specific chair. There are two chairs with swept out arms & ten without. Please enlarge all the images to check out the details.

Side Chairs:
Width – 24.5 inches, 57 cm / 21.5 inches, 54 cm
Depth – 19.5 inches, 49 cm
Height – 40.5 inches, 103 cm / 41 inches, 104 cm
Carver Chairs:
Width – 25 inches, 63 cm
Depth – 21.5 inches, 54 cm
Height – 41 inches, 107 cm
Arm height – 28 inches, 71 cm
Seat height – 18.5 inches, 47 cm
Marylebone Antiques has dated this piece to the 1880s.
Specialist in english, french and swedish antique furniture & decorative items circa 1750-1950s.
You can view the stock and full dealer information on the Marylebone Antiques Profile Page