Annual Coverage For 1893 And 1912 Is Based On Figures For 11 Months. Source: This Series Is Described In A.G. Silverman "Monthly Index Numbers Of British Export And Import Prices, 1880-1913" In Review Of Economic Statistics, Vol Xii, 3, August, 1930.
This NBER data series m04111 appears on the NBER website in Chapter 4 at http://www.nber.org/databases/macrohistory/contents/chapter04.html.
NBER Indicator: m04111
Release: NBER Macrohistory Database
National Bureau of Economic Research, Coal Exports, Prices for Great Britain [M04111GBM318NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/M04111GBM318NNBR/, August 3, 2015.