The number is probably the count of the limited edition first day covers created by Beverly Hills Cachet Service. By the way, this is noted to be Planty #19 Orange and Green Beverly Hills Cachet.
Apparently, Beverly Hills Cachet Service made these cacheted covers in red and gray, too, but are said to be very scarce and can sometimes command up to 10x the value of the orange and green variety shown in your scan.
Here's a brief history of the Beverly Hills Cachet Service:
Quote:
About Beverly Hills Cachets [Revised 3/8/12]
Beverly Hills Cachet Service produced cachets in Chicago IL from 1932 to 1938 in Chicago IL, at 10410 South Maplewood Ave., Chicago IL. The owner was Robert J. Campbell, and the featured Artist was Josef Froulo. Beverly Hills Philatelic Society (Chapter No. 104 A.P.S) at the same address was an affiliated Cachet Club.
The First Beverly Hills Cachet is #716-15 2c Winter Olympics issued 1/25/32, pictured at Mellone, Planty Vol. III, 1995, p. 19. (Monty et al., First Cachets Revealed, 2006, p. 33). Apparently Beverly Hills did not produce a Cachet for the #704/715 Washington Bicentennial Set on 1/1/32. The last Beverly Hills Cachet I have seen is #C23-32, 6c National Airmail Week Air Mail, issued 5/14/38.
Most Beverly Hills Cachets are signed with the printed note "Beverly Hills Cachet Service, 10410 S. Maplewood Av., Chicago IL," on the back envelope flap, along with the printed Serial No. of each Cover. (Mellone and Newton, 1979, p. 12-13).
References
Mellone, M.A., Mellone's Planty Photo Encyclopedia of Cacheted First Day Covers, Vol. III – 1930-1932, Stewartsville, NJ: FDC Publishing Co., 1995 (herein "Mellone, Planty Vol. III, 1995").
Mellone, M.A. and Newton, B., The Cachet Identifier of US Cacheted First Day Covers, 2d Edition, 2d Printing, Stewartsville, NJ: FDC Publishing Co., 1979.
Monty, R.A., Doyle, W.L., Goodson, M.W. and Elrod, N.L., Mellone's First Day Cover Encyclopedia of First Cachets Revealed 1923-2005, 2006 Professional Reference Edition,Stewartsville, NJ: FDC Publishing Co., 2006 (herein "Monty et al., First Cachets Revealed, 2006").