This is what we found out from our contact at Diageo...
The bottle looks to be a genuine example of Buchanan’s Royal Household. From looking at your image I would date the bottle to the late 1950s or early 1960s. Our current Queen granted a Royal Warrant to Buchanan’s in 1955, which you can see displayed as the Royal Coat of Arms and the phrase, ‘By Appointment to Her Majesty the Queen Scotch whisky Distillers.’ So the bottle must date from after 1955, and the closure and the labelling is in keeping with examples we have in our Archive which date into the 1960s, so it could have been produced any time between 1955 and the early 1960s, when the packaging was redesigned.
The history of this brand dates back to the 1890s, when James Buchanan & Co. Ltd began blending a whisky especially for the Royal Household (it was developed during the 1890s, and the first evidence that we have of the company using ‘Royal Household’ as a brand name is from 1901) and it was eventually released onto the open market for the general public to purchase
The brand is still available today, but only in Japan, where it is sold as a super premium blended whisky.