It’s actually pretty simple: the LVT (4) was entirely made of mild steel. Of the troop carriers, only the LVT (A) (2) was built of armour steel — and even then, the pontoons were mild steel.
You can recognise an entirely unarmoured pontoon because it has flat sides with vertical weld seams and one (LVT (4)) or two (LVT (2) and (A) (2)) steps in it.
The original uparmour plates are rectangular ones at front and rear, over the above. That is, if you can see weld lines and steps, but raised plates front and rear, the vehicle has the basic uparmour kit. This includes a bow plate that does not curve up over the nose. Thos was the only type used in Europe, off the top of my head, and would also be fitted to the LVT (A) (2) despite it being armoured — probably because the pontoons weren’t.
The later uparmour kit has plates that cover the whole sides, making them smooth without vertical welds and steps. The bow armour then goes up over the nose. I think this got used mainly in the Pacific.
Also, the bow machine gun was not present on early vehicles, like most seen in Europe. Very early LVT (4)’s had both hatches equally far forward, later ones had the right-hand hatch moved back a little, but this already happened before the machine gun was added.
