The standard AR A1 or A2 flash suppressors are 1/2x28 thread -- if you have one of them around, you may want to try installing it. If its 1/2x36 thread on your barrel, the A2 F/S will go on about 1 turn (finger pressure only) and stop.
But if the F/S you have came from the 7.62x39 barrel originally, that won't tell you much.
Model1 may have their own system for threading their barrels, and I have no clue re: thread vs. caliber.
I do know that their new 6.8 SPC barrels are 1/2x36 for I called them with my own question after my A2 F/S did exactly as above.
I get through to a person with an answer about 50-60% of the time I call the (847) 693-3192 number on their catalog, so you may want to try that instead of their email -- my experience is the same as yours. Not a criticism, per se, for I imagine they are a small shop with increasing business and limited staff. More power to them for I like their prices and options and have had very positive experiences on balance.
Frisco: Are you talking about the Smith or KKF brake?
My buddy and I each want one for our 6.8 barrels, figuring the .277 vs. .308 or .311 bore sizes won't matter much for a .30 cal. brake due to the increased powder capacity compared to a 7.62x39.
Now to find ammo for the darned thing.
Cheers, Otto