The GWR Roundel or 'Shirtbutton' monogram was used on GWR locos and rolling stock during the period 1934-1942. On locos it was in gilt with a black outline, positioned in the center of the tank or tender sides. At only 12" in diameter, it was unfortunately rather small, so could be somewhat insignificant in these locations, often disappearing entirely when wheathering set in.
On coaches, the monogram was carried in the center below the waist, while 'brown' vehicles and containers carried the monogram in various locations on the sides. The monogram looked considerably better on the brown stock, where it was applied in yellow. Ordinary grey goods vehicles never carried the monogram, retaining the letters 'GW'.
Click here for a downloadable pdf of the Roundel, produced by Pete Speller from the original Swindon drawing of July 1934. The drawing's annotation "1'5" from bottom of panel " refers to its position relative to the bottom of the bodyside of a contemporary Collett coach, so might not be quite right for earlier Dean or Churchward vehicles. On brown stock, it is thought the roundel did not have an outer black perimeter, so the perimeter would have been ½" yellow.