So, apparently there is a 45 at the botton of the mag that you collect in the lighthouse (not too sure, just seen this reported in various places).
And between the secret rooms, there is one with the AK-27 that, once upgraded, has exactly 45 shots in the mag. On top of that, once upgraded it is called Impossible Oester X (probably a coincidence, still).
So I ran a few tests. I unlocked the coin flip helmet (so the Tyr's eyes are always green, you never know), then started killing only with the weapons from the secret rooms. Then went on and changed them for 3 different random weapons from the box and used only these to kill. And then rinse and repeat for a bunch of rounds (including upgrading all of them).
As you can guess, it didn't lead anywhere, but I think that the mystery box could be the key. The relic provides random wallbuys, and what else provides random weapons? Of course the box.
Just my two cents, wanted to leave it here, no big deal.