Way cool.
Is this so old that ATF doesn't care about it?
No. This is a modified aircraft machinegun, general era of production around World War 1 (1914-1918). Even if it were pre-1899 it would still be a machinegun under NFA definitions.
Quite a few machineguns were sold on the civilian market prior to NFA (1934). During the several years prior to US involvement in WW1 there was considerable demand for such weapons along the southern US border where Mexican revolutionaries were raiding towns, robbing banks, and kidnapping prominent citizens for ransom to finance their causes. Lewis guns (British light machineguns), Vickers, Brownings, Thompsons, and BARs were fairly common in south Texas and New Mexico. Some large land holders and ranchers kept arsenals on hand, including automatic weapons.
Later, during the Depression era many communities throughout the Midwest US were forced to deal with the "automobile bandits", arriving in force, overwhelming local authorities, robbing banks and businesses, then fleeing in cars. Some towns built concrete bunkers in town squares and equipped themselves with BARs, Browning machineguns, and other weapons to deal with the Bonnie & Clyde and Dillinger gangs (and copycats).
The US Postal Service maintained its own "police", not unlike a small army, to guard mail shipments by truck, train, and airplane. One of the first major sales of Thompson Submachine Guns was on contract with the Postal Service. Sealed mail cars were frequently guarded by either US Marines or Postal Service officers with Browning machineguns. Even the mail clerks were required to be armed while working, frequently using WW1 surplus US Model 1917 .45 revolvers. All air mail pilots were required to be armed to protect the mail.
Armored car services took care of money transfers among banks and larger retail customers, and machineguns were commonly carried for defense against robberies.
Major industrial companies were dealing with union organizers and violent strikes, and many maintained private "police" forces with all sorts of weapons.
Anarchists, socialists, and communist groups were active in many major cities, generally acting in the belief that the time of worldwide workers' revolutions had come. Bombings and assassination attempts were not infrequent.
The era of Prohibition brought organized crime groups into existence in most cities, and violent gang wars occurred with some frequency.
These were all occurring prior to the creation of the FBI. There were no federal resources to speak of with which to address interstate criminal activities. Private detective agencies (such as Pinkerton's) and guard companies were major enterprises.
Meanwhile, the overwhelming majority of the US population remained in agricultural employment, residing in rural areas and small towns, with commerce generally limited to fairly small regions. Most of those areas had minimal law enforcement capabilities, and communications were limited to telegraph services (in some communities) or the occasional telephone.
Much different times in the United States.