Very difficult to trace, it is an invention of the late Victorian period from about c1870. In ancient auction results (The Values of Old English Silver and Sheffield Plate, London, 1906), this item is never appearing and so we can assume that it was either rare or everybody wanted to keep it.
 

HISTORICAL ORIGIN At a time when the kitchens were generally set far from dining rooms, it was of great importance that food be kept warm while being brought to the rarely heated dining rooms. Apart from the large variety of dishes that had hot water compartments so the enchanting spoon warmers were invented. Filled also with hot water in the kitchen, they were brought to the table along with the serving spoon(s) which were stored in them until being used and so kept warm. CONSTRUCTION They have always something in common, namely the fact that they stand on feet. Because of this we can assume that they were filled with hot water. The feet would prevent damage to the precious table surface. on the right: silverplate spoonwarmer, Albert J. Beardshaw, Sheffield about 1875

TODAY'S APPEARANCE They were never part of a mass production and so they rarely appear today. They are absolutely collectable and highly sought after. I think they were probably not produced in Sterling Silver. In my 20 years of collecting I only found spoon warmers, manufactured in the process of Electro Plating. They appear today in various shapes and themes. Today, we enjoy playful designs like various shells sitting on rocks, oval egg-shaped forms placed on feet sometimes with side or top handles, small boats or redesigned sauceboats and last but not least everyday's objects transformed to the purpose. on the left: silverplate spoonwarmer, Walker & Hall, Sheffield about 1885

With the beginning of the 20th Century (up until WWI) and the associated progress in relation to heated rooms these decorative and functional helpers slowly fell out of fashion.
The main production period in my view was the last quarter of the 19th Century.  

William Hutton & Sons, Sheffield about 1849 (MP Mark)

William Hutton & Sons, Sheffield about 1860