This information comes from a Chinese post on Bilibili. I’m not a naval expert so I can’t comment on its veracity, but I was curious to see what the folks here think about it.
Traditionally, the maximum thickness of the belt armour of the Nagato-class is listed as 305 mm (304.8 mm to be precise) in metric units, as directly converted from the 12-inch value in imperial/US units. This is also the value used for the Mutsu in War Thunder.
However, the author produced this image of what he claimed to be a data table by Japanese naval architects of World War II:
Note the red underlined armour values for the Yamato and the Shinano. They’re 410 mm and 400 mm, as commonly known, but the inch values are 16.4 and 16, which doesn’t conform to the 25.4 mm standard conversion! Instead, it matches the conversion used for British armour inches of 24.9 mm (for those who don’t know, British armour thickness is often listed in pounds, where 40 pounds = 1 inch of armour, but 1 inch = 24.9 mm in this case). If this value is consistently applied, the Nagato-class would instead have a 298.8 mm armour belt (about 6 mm thinner).
In fact, the 24.9 mm convention had been used in historical documents for the Ise-class, which is why it is described as having 299 mm armour, while here it is listed as having the same 12-inch armour.
Obviously, this isn’t nearly a source reliable enough for bug reporting (and even if it was, it would be splitting hairs to correct a 6 mm difference in armour thickness), but it’s still pretty interesting.