This button has a diameter of 26mm.
It is of 2-piece construction with brass back and nickel front.

The North Staffordshire Railway company was incorporated in April, 1845
with a share capital of £2,350,000 in £20 shares (117,500 shares).

From its opening in 1848 it adopted the Staffordshire knot as its emblem.
[See David J Froggatt, "Railway Buttons, Badges & Uniforms".]