For many years the British reported their velocity as "Observed Velocity". The velocities were measured by a Boulogne Chronograph which could not measure extremely small time intervals so the start screen was at the muzzle & the stop screen at 20 yds. This observed velocity of 1050fps over 20 yards was the standard loading & translated to an actual muzzle velocity of close to 1300 fps.
In some old US data you will see even lower velocities listed. We measured during this time frame over 40 yds. With the advent of electric Counter Chronographs the required time intervals were shortened so measurements began to be taken much closer to the muzzle so naturally show higher velocities for the same loading.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra