Bulgarian flash supressor legal??

Found a 4 pc. can type Bulgarian flash supressor in the shop from years past. Is the thing legal and what is the thread size? I recall years ago when they first came into country there was talk that BATF might call them a "sound supressing device". I just want to know If there is jail time in my future if I mount it on my ak47

If you go, thousands of others'll be on the bus with you.

Jinx do you by chance have another?

I'll look. Been cleaning out the shop and found 3 guns and a footlocker ful of mags and there has to be more stuff burried.
Thank all,