Greetings Odinsravin - my fellow Norseman,
I have both makes/models you mentioned.
My Brno 602 is a .375 and the magazine holds 5 rounds.
One of my CZ 550 Magnums is a .458 Lott (custom build but the magazine capacity was not modified in any way).
Today, as a .458 Lott, it holds 5 in the magazine.
It began life as a .375, and back then, it used to hold 5 of those in the magazine, as well.
I suspect you made your bet regarding the typical H&H diameter / belted cartridges.
But if not, I also have a CZ 550 Magnum in .500 Jeffery and it holds 3 in the magazine.
Likewise, I don't think you're asking about non-magnum Model 550 capacity.
But if I'm wrong, I have a CZ "Standard" 550 in 6.5x55 and no surprise, it holds 5 in the magazine.
Cheers,
Velo Dog.