Gekasan_RU
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I think the factory number is actually the registration date on the forum, which is absurd, but here's how it works: 5 4 3_1 – the registration date on the Nephalem forum is 05/31/2012. So, 3_1 equals the 31st, 5 equals the 5th month, and 4 equals 2012, because the era of the 4th sun ends in 2012. Something like that.
By the way, that's a very good guess, I like it!)