I agree with what MrG said. If your WASR has a muzzle nut spot welded on then it is covering threads. During the ban years muzzle threads had to be permanently covered and the spot weld was good enough. If there were no threads then there'd be no reason to weld on a thread cover. I'd be shocked if it does not have threads, never heard of a WASR that didn't.
Got my WASR over 4 years ago and it came with the welded nut. I cut it off and bought the Tapco slant brake and Tapco cage flash hider. I like the flash hider better. If you still want the 74-style aftermarket flash hider I don't know much about them but remember the originals have a different thread size and direction than the 47 and are for .223 cal bullets. I believe they make knockoffs with the proper threads and bullet opening but be sure. Whatever, BE SURE it is designed for a 30 cal bullet not the 74's 22 cal!!!!
To be compliant with 922r parts count (Fed law) you should buy a muzzle attachment made in the US, not imported unless you're sure the extra foreign part won't throw you into non-compliance. By cutting the weld my WASR was non-compliant until I put on the US made Tapco part.
ETA: more info...
I've heard some nuts are hard to get off after cutting the weld. I soaked mine overnight with penetrating oil before I cut the weld and it came off pretty easy. Just remember it unscrews the opposite way we're used to. If the muzzle attachment you choose is loose, you can file a little on the back of it so it fits snug then clicks into place. It's pretty normal for slant brakes to rattle around but I didn't like it.