 This meant that the cylinder castings were split into two sections, with the left and inside cylinders forming a single unit that would be bolted onto a separate right-hand cylinder casting.Holcroft’s new cylinder block design also incorporated a separate saddle casting, a configuration that allowed the N1 cylinder patterns to be used with any diameter of smokebox, creating a standard component with potential applications on future locomotive designs.The construction of the prototype at Ashford railway works was approved by the SECR management to commence in 1919.