The Monitor was a variant of the Colt Automatic Machine Rifle, which was the commercial version of the BAR marketed by Colt in a number of variations between the early 1920's and WWII. The factory designation for the Monitor was R80, and it was introduced in 1931 in response to reports that police roadblocks equipped with .45 ACP Thompson submachineguns were being routinely run by large heavy beer trucks. The .30-06 Monitor was quite capable of splitting engine blocks and stopping the trucks.
The Monitor was geared specifically for the law enforcement market, and was also the first offical FBI fighting rifle.
The Monitor was a direct descendant of the Model 1918 Browning Machine Rifle, but there were a number of key differences:
The receiver was fitted with a pistol grip and ejection port cover, it was some 4 to 4.5 pounds lighter, and had a barrel that was only 18" long.
The compensator you note is a Cutts compensator made by the Lyman Gun Sight Company, and made the Monitor surprisingly controllable in full automatic more. Cyclic rate of fire was 500 rounds per minute.
The Monitor was produced up to about 1940, with about 125 total being produced. A rare version in 7.65mm Argentinan cartridges BAR type guns was shipped to Argentina:
June 13, 1933, seven 7.65mm guns were shipped to F.C. Rasetti y Cia
September 20, 1933, two 7.65mm guns were shipped to Angel Muzzioe Hijos,Lda.
December 12, 1933, two 7.65mm guns were shipped to F.C. Rasetti y Cia
April 11, 1934, seven 7.65mm guns were shipped to F.C. Rasetti y Cia
June 25, 1934, one 7.65mm gun was shipped to F.C. Rasetti y Cia
Photo and information from:
DCB Shooting Supply