They comprised the last ten of the twenty locomotives of Lot 64. The first ten locomotives of this Lot were built to the broad gauge as the 3501 class.
Allcock, N. J.; Davies, F. K.; le Fleming, H. M.; Maskelyne, J. N.; Reed, P. J. T.; Tabor, F. J. (1968) [1951]. White, D. E. (ed.). The Locomotives of the Great Western Railway, part one: Preliminary Survey. Kenilworth: RCTS.
Tabor, F. J. (August 1959). White, D. E. (ed.). The Locomotives of the Great Western Railway, part six: Four-Coupled Tank Engines. RCTS. ISBN0-901115-36-3.
Holcroft, Harold (1957). An Outline of Great Western Locomotive Practice 1837-1947. Shepperton, Surrey, UK: Ian Allan Ltd. ISBN0-7110-0228-2.