If you are talking about a SIG AMT there were no muzzle brakes intended specifically for them. There were a number of different types of flash hiders used. The flash hiders were installed usually by the importer so its a crap shoot. The most common were the FN49 type but I've seen a couple guns with HK types too. The threads are 15mmX1 I believe...or whatever the standard thread for the HK91. My rifle had the FN49 type. Many rifles had no flash hider. If you are looking for an actual muzzle brake you will probably have to get something made. Estes Adams made some threaded flashhider/muzzle brakes that duplicate the original 510-4 type. You might try contacting him if you want one of them.
hope that helps some.
The 510-4 has the integral brake/flash hider but all rifles imported into the US legally (except a very few marked 510-4) were imported with no flash hider. The barrels are cylindrical from the front sight base forward. I can get some pictures when I get home from this show on monday. All the US guns were marked AMT rather than 510-4. There was a very small batch of guns imported with the 510-4 marking and the standard barrel with the flash hider/brake but the numbers were less than 10 and from what I've seen and read the guns were in the same boat as the G series FAL's imported from Belgium. They had full auto receivers with semi auto only lowers installed. I've only seen one of the real 510-4's from that batch and I didn't get to take it apart to see if it was cut for the trip sear. There is a good bit of info on the US guns at biggerhammer.net . Their info includes a list of importers and some of the different characteristics the US guns have.