AAMG.

This is coolbert:

 From a variety of Internet sources some images from the Great War [WW1] - - machine guns as used by ground forces in the anti-aircraft role.

 Especially from that period of 1917 onward:

 "As aircraft started to be used against ground targets on the battlefield, the AA guns could not be traversed quickly enough at close targets and, being relatively few, were not always in the right place (and were often unpopular with other troops), so changed positions frequently. Soon the forces were adding various machine-gun based weapons mounted on poles. These short-range weapons proved more deadly"

 German blitzkrieg tactics as first used at Riga, 1917, always incorporating the three elements of:

 * Movement forward by special units of Storm Troop [Sturmabteilung] using infiltration tactics.

 * Poison gas delivered on target by artillery shell.

 * Ground strafing aircraft.

 That means to combat the ground strafing aircraft [often flying nape-of-the-earth] the machine gun, mounted either with an ad hoc apparatus or atop a pole. That pole allowing for total three hundred sixty [360] degree coverage if and when the target presented itself, engagement from any direction possible.

 Heavy machine guns the weapon of choice during the WW1 era. That anti-aircraft-machine-gun [AAMG] best defined as:

 "Heavy machineguns"

 "A machine gun is rated heavy when it cannot be carried and operated by a single man. The class can be extended to all machineguns with a recoil too powerful (because of caliber or rate of fire) to be controllable without a mount."

 Heavy in my mind defined as a machine gun of fifty [12.7 mm] or greater even or having a water-cooled jacket for the barrel or the entire apparatus [gun, tripod, ammo] not being able to be carried by ONE man. 

To the images. Posed I would think in most instances and not real coverage of combat action:

British troops manning a Vickers AAMG ad hoc mounted.

British Lewis gun with bipod and air-cooled barrel.

French St. Etienne gun operated in an ad hoc manner and NOT aiming at a ground strafing aircraft.

American gun crew manning what I believe is a Hotchkiss AAMG.

This is obviously a posed picture? Gun is mounted on a pole in this case.

Maxim guns mounted on poles. Total traverse available if needed.

This Maxim gun able to engage low-flying ground strafing aircraft. An ideal. 

 A "clump" of massed AAMG even if not successfully engaging and shooting-down an approaching enemy aircraft, force that adversary to fly higher and take evasive action, negating an efficient strafing or bombing mission.

 coolbert.