Open top.
But that is a good question.
Open versus closed top receiver should not affect the feeding from the box magazine, but it may affect the ease of even ability to top load.
I have adjusted the feed lips on the box to be able to top load. The open top is a plus here.
I suspect with the closed top receiver you may have to bottom feed using the hinged base plate. Not a big deal and makes adjusting feed lips less critical.
I run Hawkins Hunter mags on a different rifle and use a closed top Mausingfield in that application.
Both are long actions. Both are hunting rifles.
Horses for courses.
I went to the Wyatt extended box for longer COAL to be able to run Barnes 212 Bore Rider bullets which of course Barnes has now forgotten how to make.
Given the design of the bore rider bullet, I built these rifles with a very short freebore which of course does not work very well with other bullets.
Shortages suck.