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Carriage letters - those I remember were mostly paper labels inside the windows. I seem to remember on the SR boat trains there was a letter and a number e.g. A4. In about 1973 the LMR had yellow plastic adhesive coach letters on new Mark 11 e/f stock. On the GE section of the Eastern the class 309's had fiberglass letters painted grey that slid into brackets on the coach exteriors.