The mintage figure for 1917 is misgiving; due to World War I Britain came off the gold standard and in 1917 gold was scarce. Hence the entire mintage was retained by the Bank of England and bar a few coins they were all shipped to the US to pay for WWI debts.
After passing the Gold Reserve Act in January 1934, these were all melted down, hence a very small number of coins exist today. The NGC population report shows 20 examples, making this one of the great rarities of London mint sovereigns.
