Hogg appears to have this covered in his description of the development of the 25pdr.
A 3.7" tube firing a 25lb shell was planned for the 18pdr jacket, to convert 18pdr guns into "gun-howitzer" variants. However, the calibre had to be reduced to 3.45" to allow the original 18pdr breech and mechanism, the conversion being officially approved in LoC, Equipment, 26 August 1936 as "Ordnance QF 3.45in Mk I". This governed conversion of 18pdr Mk IV guns to box trail Mk III or split trail Mk V carriages.
In LoC 11 February 1938, the nomenclature was changed to "Ordnance QF 25pdr Mark I", though it was commonly called the "18/25 pounder". Over 1,000 were made.
The photo is the one at Firepower Museum, which I've posted before on a 25pdr thread.
(In Africa and Italy my father fought alongside an ex-gunner who in May 1940 had helped push his 18/25 into a river before heading for the Dunkirk beaches. It seems many of the 18/25s suffered similar fates.)
Tom.