One millimetre? That's less than the width of these text characters.
I'm not sure the game can even render that.
This is the CoH model from mods:
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Like I said, you can barely notice the extra slabs of armor unless you zoom all the way in and even then it's hard to distinguish.
The shorter 75mm gun in the 76 gun turret is the biggest visual communication.
Of course the 76mm gun equipped Sherman Jumbo in CoH's mods are a little hard to distinguish from the regular 76mm Sherman as well unless they're right next to each othere where you notice that one of the 2 is thicc boi.