@KurtGodel7 said:

“Light gun” fits the units you already have: light gun, field gun, heavy gun, getting stronger in that order. “Light mortar” would stand on its own, with no heavier mortar to compare it to.

I see that you are using "field" as a "medium" classification, but some terms like "field gun" can change wildly in their meaning. For example, a field gun can be something like the Soviet 76-mm divisional gun M1942 (a light gun) or the American 155 mm gun M1 (a heavy gun) or both depending to whom you ask.

Also the term "gun" is questionable. In classic artillery classification, that may indicate a relatively long barrel, akin to the meaning of cannon. If you have light gun, field gun and heavy gun and no other artillery units, are you excluding howitzers this way? I'm not sure if it would be advisable using the term "artillery" instead of "gun"?

Maybe ask Claude what would be the best way of dividing all World War 2 artillery above infantry mortars but below 200 mm calibres into 3 categories only, lighter to heavier (if this is what you actually want).

@Vendetta said:

How about Infantry Gun then? That was a pretty common term and it creates less confusion with the unit’s purpose.

After hashing this out with Claude, I've decided on light gun. I wanted a name which would set expectations nice and low. "Anti-tank gun" fails in that, as you pointed out, because it implies that it does something special against tanks. Putting a word like "infantry" in the name might imply it's a type of infantry.

"Light mortar" would have been a good name too. But when I bounced the idea off of Claude, it gave me back the following.

I’d still pick light gun.

A light mortar is an infantry weapon. Light mortars like the 50 mm and 60 mm were carried and fired by ordinary infantry squads. That suggests the unit should cost MP, which undercuts the reason it exists: it’s the cheap unit that doesn’t need MP. A gun is equipment, which explains the PU-only cost much better.
Mortars exist to support infantry. Players would expect a light mortar to give infantry a bonus, the way the field gun does, and this unit gives none. A light gun has the same issue, but less of it, because guns already come in a ladder where the smallest can reasonably be the weakest.
“Light gun” fits the units you already have: light gun, field gun, heavy gun, getting stronger in that order. “Light mortar” would stand on its own, with no heavier mortar to compare it to.

Light mortar does have one thing going for it: it’s a very modest weapon, so nobody will expect much from it. But “light gun” already does that, and it avoids the two expectations above.

Tanks

Light tank: attack 2, defense 1, cost 5 PUs
Tank: attack 3, defense 3, 2 hitpoints, cost 8 PUs

Improved light tank: attack 3, defense 2
Improved tank: attack 4, defense 4, 2 hitpoints

Britain's tank techs
Churchill tank (a heavy tank): attack 5, defense 5, 4 hitpoints
Centurion tank (improved heavy tank): attack 5 + 5, defense 5 + 5
improved tank
Comet tank: Britain's tanks attack and defend on a 5.

Japan's tank techs
Medium tank: allows it to build medium tanks
Improved tank

Italy's tank techs
Medium tank tech: allows it to build tanks

USA tank techs (both halves)
Pershing tank: allows the US to build heavy tanks. Attack 4 + 4, defense 4+ 4, 4 hitpoints.
Improved medium tanks
Improved light tanks

Soviet tank techs*
KV-1: Allows the USSR to build heavy tanks. Attack 4 + 4, defense 4 + 4, 4 hitpoints.
Improved medium tanks
Improved light tanks
IS-2 (improved heavy tanks). Soviet heavy tanks attack and defend on a 5 + 5.
Simplify the T-34. Soviet medium tanks now cost one less PU

German tank tech
Panther: Allows Germany to build Panthers. Attack 4 + 4, defense 4 + 4, 4 hitpoints.
Speer's industrialization: every other turn, Germany gets a free Panther at each of its SPCs.
Improved medium tanks
Improved light tanks
Improved Panther (E-50): Germany's Panthers attack and defend on a 5 + 5
King Tiger: allows Germany to build King Tiger tanks. A King Tiger attacks and defends on a 4 + 4 + 4 and has 6 hitpoints.
Improved King Tiger (E-75): King Tigers attack and defend on a 5 + 5 + 5, and have 7 hitpoints.

Note that heavy tanks (Churchill, Centurion, Panther, Pershing, KV-1) and King Tigers are special units. These cannot be purchased with PUs, but can only be produced at SPCs. I acknowledge that Germany has the best SPC tanks in the game, just as Britain has the best tanks purchased with PUs. Germany needs the better SPC tanks to balance Soviet conscription techs. Conscription 4 results in the Soviet Union getting +4 infantry per turn for five of its factories (assuming none of the five factories have been conquered). Assuming both Germany and the Soviet Union are maxed out on tech, and assuming the Soviet Union still holds Moscow/Stalingrad, then every turn the Soviet player gets 20 free infantry. Germany would get 5 free Panthers every two turns. If you're looking at free units received every two turns, that's 40 Soviet infantry compared to the 5 Panthers. In terms of raw hitpoints received this benefits the Soviets by a 2:1 ratio. The superiority of the King Tiger over the IS-2 helps balance out some of that 2:1 ratio.

Infantry

Tier 1: China. Conscript. Attack 0, defend 1. Cost 1 MP
Tier 2: Italy, USSR, Japan. Infantry. Attack 1, defend 2. Cost 1 MP and 1 PU.
Tier 3: UK, USA, Germany. Infantry. Attack 2, defend 3. Cost 1 MP and 1 PU. The USSR can have this unit also, called "Revolutionary Guards," cost 1 MP and 3 PUs.
Tier 4: Sturmtruppen. Germany. Attack 3, defend 4. Cost 1 MP and 3 PUs.

Japanese Marine-equivalent. Attack 2, defend 2. Cost 1 MP and 2 PUs. Special: gets to fire on the first round of amphibious assaults.

Marine. America Pacific only. Attack 3 defend 3. Cost 1 MP and 2 PUs. Special: gets to fire on the first round of amphibious assaults.

Tech for Japan: Tadamichi Kuribayashi. Causes Japanese infantry to attack on a 2 and defend on a 3. Japanese Marine equivalents attack and defend on a 3. Two stars.

All units described above can receive support on both offense and defense.