Check the bottom right hand corner of the label, it will have a number reference which normally refers to the month and year. Our contact at Dewar's tells us it looks probably c1910s. Below is more information - courtesy of Dewar's:
DEWAR’S OLD HIGHLAND / SPECIAL LIQUEUR WHISKY
Dewar’s most famous and enduring product, White Label, was not introduced until approximately 1899. Prior to that Dewar’s produced a range of blended whiskies and by the mid to late 1880s had developed a large portfolio of sophisticated blends. By the 1890s these largely consisted of Old Highland Whisky which was available in various guises, including Special / Extra Special / Special Liqueur and Extra Special Liqueur. All of these were created from different recipes, containing different ages of whisky, and were therefore designed to meet different price points and markets. Our most expensive products contained some very, very old whiskies, although in the absence of the old recipe books it is difficult to determine the precise differences between each of these products.
A promotional advert produced by the Company in 1894 described our ‘Special Liqueur’ whisky as: ‘the very finest Whisky, of the greatest possible age, largely used for medical purposes, and being prescribed, at present, in preference to the oldest Liqueur Brandy.’ In a price list of 1897 it was described thus: ‘Our Special Liqueur – as supplied to Her Majesty the Queen [Queen Victoria] – we believe to be the Finest Whisky in existence.’