Due to the limitations of early mechanical chronographs, Pre-WWII shotshell velocity was recorded as the average velocity over 40 yards.
In pre-WWII Winchester catalogs, "3 dram equivalent" trap load, (#7 1/2 shot), velocity was listed at 850 fps. With the coming of electronic chronographs, manufacturer advertising circulars began listing the "muzzle" velocity of trap loads at 1200 fps.