Arsenal Saiga SGL20-41 vs. SGL20-21. whats the difference?

I'm browsing auctionarms and gunbroker. I find two different Arsenal Saigas. One has the 74-style brake attached. The other, however, has the muzzle nut instead. One is the SGL20-41, and the other is the SGL20-21.

Basically:

-what are the main differences between the two models?
-is the 'muzzle nut' removable, and thus allows for the installation of a 74-style brake? (is the roll/retaining pin present and ready to go?)

Googlefu has failed me. Any quick help is appreciated.

Already looked. Didn't see it listed. Not even Arsenal's site gave me any real info.

That is what I'm talking about. What the hell exactly is that muzzle nut, and does it come off to allow a compensator or brake? Like, is it held by a roll pin like a brake would be?