Great Britain, a stamp issued probably 1902, likely to be SG 215. The cancel is for Walsall, which at the time was a very industrial town in the middle of the country, just west of Birmingham. The 1/2d stamp came in a variety of blue-green shades, also in yellowish green. Yours is one of the blue-green shades.
De la Rue first printed this stamp of King Edward VII in blue-green (SG215/216). In 2004, the colour was, purposely, changed to yellow-green (SG217/218). In 1911, Harrison made provisonal printings in yellow-green (SG267-271). They also perforated those stamps 15x14 (SG279/279a). As Tim H suggest, yours is SG215 (or 216) in the blue-green colour.