Millibar (mb) = is today HectoPascal (inces of Mercury in USA - 29,92 in / 1013.25 mb at + 15 Deg C = ICAO Standard Pressure <Average>,
Standard is used for cruising, i.e. Flight Levels (ie. FL300 or above) are cruising level of todays Airliners at 29,92 in or 1013.25 mb.
Change from Local Pressure to Standard usually was about 5000 - 7000 feet, or when max deviation from normal no longer need be taken into account.
3000 feet cange used in old days resulted in accidents, because high winds blowing over mountain ranges create (often) huge deviations from true altitude.
QNH or QFE. Q code can be found in Lw flying missions reports.
https://en.wikipedia.org/wiki/Q_code