A vintage handmade/mouth blown Holmegaard glass decanter. The design is attributed to Jacob Bang. This example is smokey grey glass. 
Holmegaard first started making the Kluk Kluk in 1840 and revived the style in 1960, so circa 1960s or 1970s vintage. It is known as the Kluk Kluk (in Danish) or Glug Glug (in English) decanter due to the sound it makes when it is poured. Distinctive pinched middle, squarish shaped (low capacity - designed for a smaller amounts of the top-notch spirits kept at a colder temperature). Original round stopper. Stopper has ground down end, and the inside of the neck is ground down too so it forms a good seal.
A tall version of the design - 30.5cm tall x 10cm wide max. Very good lightly used condition. No chips, sharp edges, nibbles etc. A couple of tiny bubbles and a few marks towards the baseline. Just mentioned for accuracy - but actually a very good clean example, perfect for display, use, or as a prop. 
Iconic mid century Scandinavian design!
Free P&P to the UK.