The double Kylchaps were fitted to the entire Gresley pacific fleet in the late 1950s (those that didn't have them already). That was towards the end of steam on the East Coast, and was carried out explicitly to improve performance and save coal for which a sound business case was made at the time with strong support from senior management including Gerry Fiennes (of "I Tried To Run A Railway" fame). The measure was very successful with A3s and the thereotically more powerful A4 being able to be used interchangeably on 90MPH express diagrams day in day out, even successfully substituting on many failed diesel diagrams during the changeover period.
Whilst the A4 streamlined casing had always been known to be a great smoke lifting device, the soft Kylchap exhaust on the A3s proved much more prone to drifting at speed and that was considered a safety problem, particularly for sighting signals. My father, Peter Townend, who was shedmaster at Top Shed at the time, made the suggestion of the German style deflectors as he had observed the type always performing well during his travels in that country. A further advantage was the design didn't extend down to footplate level so they were no obstacle to crew and depot staff safely walking around the front of the locomotives. He sent some photos of the German designs to Doncaster's drawing office who quickly produced a design that was applied to all subsequent A3s visiting the works for overhaul. Many locos received their deflectors at the same time as their Kylchaps.
Love or loath their appearance, the double Kylchap and deflector arrangement was an undoubted success from a performance, economy and safety perspective. The performance and safety considerations in particular are still important for heritage main line operations on today's busy high-speed railway and whilst retaining these features for good operational reasons at least Flying Scotsman is now due to appear in a historically accurate late BR livery.
