Funny I was just looking at the internal box magazine of my BRNO 602 in 358NM and my BRNO 602 300WM and there is a difference on how the box mag is built!
The 358NM has 2 bump outs on either side, while the 300WM does not, the followers are very similar , but has minor machining difference's?
All the conversions I have handles, just used a standard magnum followers and box mag.
A great cartridge , very accurate , wish Serria made a match 250gr bullet though?
Good luck! Dale Z!