Just to add some general information to what Liz has already provided, an equerry “is essentially a military [appointment] which is invariably filled by a service or retired officer of the Armed Forces”. (Ronald Allison and Sarah Riddell, Royal Encyclopedia, London: Macmillan Press, 1991, p. 176.) For example, HM The Queen has two equerries – a senior/permanent one known as the Deputy Master of the Household and a junior one known as the Junior Equerry. The Junior Equerry, whose appointment lasts three years, wears his service uniform while on duty. He is in “close attendance upon Her Majesty at many of her public engagements”, whether outdoors (say, at a public function) or indoors (say, at an investiture). (RE, p. 176.) Unfortunately, the entry for ‘equerry’ in this publication does not seem to mention who is an assistant to an equerry or what his duties are.

If your individual in question died in 1948, then he was likely an equerry’s assistant during the reigns of King George V (1910-1936), King Edward VIII (1936), or King George VI (1936-1952). Take a look at thePeerage.com (at http://www.thepeerage.com/index_royal.htm) where you’ll find an “Index to the Royal Household”. This index lists various office holders, such as Equerries, Aides-de-Camp (whose functions are similar to Equerries), Private Secretaries, and more.