The 40mm AA gun is usually called a Bofors, after the Swedish designers. The same basic gun was used in either single mounts, twin mounts, or quad mounts by both the US Army and Navy, and also by many allies during WW2. The Air Force AC-130 gunships still use the 40mm single gun along with a 105mm howitzer and one or more 20mm Vulcan "Gatling" guns.
A friend of mine used to be associated with that operation, and reportedly they depended on the US Navy Battleships converted to museums as a source of spare parts to keep the 40mm guns operational. Your rig should have four 6.00x20 pneumatic tires and two folding outriggers on the carriage and four leveling jacks. Overall length 19 feet, width 6 feet, height 7 feet. Road clearance 14 inches and weight 5,549 lbs.
These were towed by a 6x6 truck. Rate of fire was 120 rounds per minute, with maximum effective range of 3,000 yards with a muzzle velocity of 2,870 feet per second using a 2 pound projectile and .65 to .72 pound propelling charge. (In case you needed reloading data!) Ammunition was loaded into the gun on four round clips. There are several tech manuals related to the guns and carriages and I can provide numbers if you want to try to track them down. The Army referred to the single as "40mm Automatic Gun M1 on carriage M2 or M2A1". Therefore I suspect your data plate refers to the mount, not the gun. Hope this helps.
edit: forgot to add link.
oldguns.net/cgi-bin/f2f/f2f.pl?http://oldguns.net/q&a12_00.htmA little more than half-way down the page.