Hello. A4 double chimneys: 4 A4s were built from new (1938) with double chimneys - 4468 (60022), 4901 (60005), 4902 (60033), and 4903 (60034). The remainder were built with single chimneys and were not converted until 1957/58 by British Railways. The loco in the Northumbrian set is I suspect 60003 Andrew K. McCosh which was converted in July 1957. However; you can't simply convert the chimney and add the later 1956 BR logo to the tender as 60003 had a number of tender changes. June 1952 was fitted with a streamline non-corridor tender, January 1954 fitted with a corridor tender, September 1955 fitted with a streamlined non-corridor tender. There were also 2-types of corridor tender coulped to the A4s; the original 1928 corridor tenders fitted to the A1s and A3s and the 1938 corridor tenders designed specifically for the A4s. 60003 was coupled to a 1938 corridor tender in the period of the Northumbrian set which I suspect you have. So 60003 in the set is correct for January 1954 to September 1955. If you want to convert it to double chimney you also will have to change the tender to a 1938 corridor one. Hope this helps.