This is the barrell insulator patented by William Fothergill Cooke in 1842. The insulator is made of brown glazed stoneware, and this specimen probably dates to around 1845. They were attached to a piece of wood with the U-shaped iron seen in the photo, this piece of wood was in turn mounted to the pole. One drawback with the design was that the entire length of wire had to be put thru all the insulators. They were in use into the 1850's when being replaced by other patterns.

This is a quite tiny insulator, being ca 70 mm in length and ca 45 mm in diameter.