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 m04103 appears on the NBER website in Chapter 4 at http://www.nber.org/databases/macrohistory/contents/chapter04.html.
NBER Indicator: m04103
Release: NBER Macrohistory Database
National Bureau of Economic Research, Grains and Wheat Flour, Price, Imports for Great Britain [M04103GBM318NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/M04103GBM318NNBR/, August 3, 2015.