File:40mm ammunition display board DSC 0995 (13374969213).jpg

From the "mk2, mod 0" I'd guess this is a U. S. Navy or Marine object. Rather than U. S. Army. For guns and tanks and that sort of discrete, utilitarian, object, the U. S. Army uses "M1A1" kinds of nominclature, which mean, essentially, the same as a Navy "Mk 1, Mod 1". M1A1 is a version of the Abrams main battle tank, for example, made between the M1 and the M1A2. M4 was the familiar WWII Medium tank that most of the world knows as a Sherman. Probably even the Army calls them that, now, but "M4 Medium Tank" is the official name. Using Civil War generals (Sherman, Lee, Grant, Stuart) as names for US tanks in WWII was a UK innovation that the US only slowly adopted.

The weakness of this system is that there's a M1 or Mark 1 or Mk 1 of everything- so when you write "Mk 1" it could be a display board, inflatable dinghy or atomic bomb. It *really* could be an M1 Abrams tank, an M1 Garand semi-automatic rifle, M1 Carbine or M1 Helmet, to name one modern and 3 WWII objects. M3 is about as bad, there being an M3 armored car, M3 medium tank, M3 light tank and the M3 version of the Bradley Fighting Vehicle.

Once you're out of the single digits, it gets easier. M-16 is a familiar assault rife (now upgraded and known as an M-4, M4A1, etc.). M 997 is the big 8 X 8 truck, like an airport fire truck or a concrete pumper. Mark 82 is a 500 lb bomb using the "Aero 1A" shape that Douglas Aircraft perfected in the 1950s. Mk 57 *is* an nuclear bomb, now renamed B-57 for clairity, leaving only Mk 57 vertical launch assemblies that hold missiles on ships OR Mk 57 submarine-laid anti-submarine mines. The advantage of numbers is that they can be easily organized in tables. Using names that honor people, places or events makes for more complex book keeping. But makes it easier to keep things straight, if some planning goes into which names are used for what. (Civil war Generals for tanks. State names for battleships, city names for cruisers, hero's names for destroyers, Army, Air Force and Marine bases.)

But you can overthink anything, and the exhaustive use of fish names for US submarines (Tunny, Wahoo, Perch, Balao) gave way to statesman's names for ballistic missile subs (Washington, Jefferson, Lafayette, Ethan Allen) and city names for attack subs. (Los Angeles, San Diego). Except for the USS Jimmy Carter, because he'd been a submarine captain. Most Republican presidents get an aircraft carrier, but I wouldn't hold my breath for the "George W. Bush." Or The "Lyndon Johnson", "Bill Clinton" or "Barak Obama". I think there is a case for the three Democrats, and I'd even trade those three for the "W". But the "Richard Cheney" ain't gonna happen, even if it could get through today's House of Representatives. Maybe.