efficient than 2001, due to a lower cylinder clearance volume and because the stepped-cam cutoff modifications made to No. 2001 reduced economical working relative to the infinitely variable cutoff of No. 2002. Consequently, the following locomotives were built with piston valves. By June 1936 the third engine had been produced: No. 2003, Lord President, based on the design of No. 2002 but with the external design modified to resemble the Silver Link locomotives. (see LNER Class A4). The locomotive weight was reduced to . The wedge-shaped front was found to lift the engine's smoke clear of the driver's view; No. 2002 was altered to this form in 1936, and No. 2001 in 1938. Three further locomotives, Mons Meg, Thane of Fife and Wolf of Badenoch, were under construction at Doncaster in 1936. No. 2004 was fitted with an experimental butterfly valve blastpipe bypass, manually activated to prevent fire-lifting at high cutoffs. This was later replaced with a plug valve and higher bypass pipe diameter, but both designs had problems with sticking due to carbonised deposits. No. 2005 lacked the Kylchap double chimney of the rest of the class, and No. 2006 had a different boiler design, with a longer combustion chamber, and firebox heating area and volume of and respectively with a Robinson superheater. The production series was completed in 1936. Testing, performance and service Shortly after being put into service, on 19 June, No. 2001 was tested with a 19 bogie carriage train of 649 tons on a return journey between Kings Cross, Grantham and Barkston; the locomotive hauled the train at an average speed of over 50 mph, with peak speeds of over 70 mph. Drawbar pulls of around 6 tons at around 60 mph were recorded, representing a peak output of more than 2000 horsepower. In late 1934, the locomotive was sent to Vitry, France for static testing. Point contact on the infinitely variable cams of No. 2001 led to cam damage after ~10,000 miles of service, resulting in the replacement with stepped cams giving six steps of cutoff (12, 18, 25, 35, 45 and 75%). By 1939 No. 2001 had had its rotary-cam poppet-valve valvegear replaced with Walschaerts gear. In 1939, E. H. Livesay reported from the footplate on the performance of No. 2004 Mons Meg on the early morning 'Aberdonian' non-stop from Edinburgh to Dundee – a train with gross weight of 320 tons. Peak