This is the first time I heard about one of the BB's being refitted with dual 5 inchers.
The most famous HMS Agincourt was a battleship present at the Battle of Jutland. She was a unique vessel, laid down by Armstrongs at Newcastle upon Tyne as the Brazilian Rio De Janerio in September 1911. The order was cancelled in 1912, but the design modified and sold to the Turkish navy in January 1914 as Sultan Osman I. She was completed in August 1914, just before the First World War. The war broke out before delivery, whilst she was undergoing trials, even after the Turkish crew had arrived to collect her and she was seized by the British Government for incorporation into the Royal Navy.
At the same time a second Turkish battleship was also taken over - Reshadieh (renamed HMS Erin). This caused much ill-feeling in Turkey, particularly as these ships had been partially funded by public subscription. This was an important factor in turning public opinion against Great Britain, and a major cause of Turkey (and its Ottoman Empire) joining the war on the side of Germany and the Austro-Hungarian Empire against the Triple Entente of Great Britain, France and Russia two months later.
She was an unusual ship in having seven main in-line turrets. She was poorly armoured in comparison with her armament, having just 9 inches maximum belt thickness compared with 12 or more which would have been more appropriate for her armament. She would have been classified as a battlecruiser but for her low speed. By the time that she was built, her 12 inch guns were becoming obselete - most capital ships under construction having larger calibres. Her outline design was produced by the chief designer of Armstrongs, Eustace d'Eyncourt, in his hotel bedroom in Brazil.
Modifications were made before she was commisioned in the Royal Navy. In particular a flying-off deck for seaplanes was removed. She was part of the First Battle Squadron at Jutland, which she survived unscathed. She was reallocated to the Second Battle Squadron in 1918 and decommissioned in 1919. After unsuccessful attempts to sell her to the Brazilian Government she was recomissioned as a depot ship before being decommissioned again in 1921 and scrapped in 1924.