This quite plain looking insulator is known as Neale's pattern line insulator.
Mr Neale was the Head of Signals and Telegraph for the North Staffordshire Railway (NSR), centred around Stoke-on-Trent and introduced this pattern in about 1880, continuing in use until about 1965.
Bullers coded this E.I.458 in their catalogue of the late 1880's
Taller and thinner than a Standard Post Office pattern cordeaux early examples were cemented to pins and later ones were changed to a screw thread.
Image courtesy of Jim Gough