It was an issue item for the M1919 series and also the M1917 and was listed in the technical manuals of the time. An M14 tool roll was issued for the .50 Cal BMG. Nobody seems to know exactly what went in the roll. but the most common small spare parts seem to be:
Firing pin and spring
Barrel headspace spring
Assortment of small springs and maybe a recoil spring
Broken shell extractor
Possibly cleaning equipment, the roll will hold a complete M10 cleaning rod and handle and bore and chamber brush.
Just about any other spare part the crew could lay their hands on.
Repair parts were lsited in the TM as a 15 day allowance for organizational maintenance support depending on the number of weapons in the unit. In the 1960's the Army decided that no spare parts would be kept on hand in units. The spares and repair parts were then retained at a higher level maintenance shop and issued as needed.
There was also a bag for a complete spare bolt that was issued. At one time a tool box was also issued with the gun. Early ones were wood mostly for the M1917 and later ones were metal.