Traditional symbols for 1000 (not one million) board feet, a unit of volume for timber equal to 250/3 = 83.333 cubic feet or 2.360 cubic meters. "BM" stands for "board measure."
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}