The byonet lug is NOT the grooves on the back of the flash suppressor. It's the lug that sticks down under the front post of the front sight base, and if you don't have a lug sticking down under the front sight base, or if you do have one and grind it off, you're halfway to having a post ban legal upper.
The other half would be the flash supprssor itself. You wrote in an earlier post that the suppressor was permanently attached. Are you sure? If it is indeed a FLASH SUPPRESSOR, for a preban gun, there would be no need for it to be permanently attached, nor for that shiny washer.
If in fact it is simply threaded on, you can make the unit post ban legal by replacing the flash suppressor with a muzzle brake that has been permanently installed, and then you can use either top end on either of your guns.
To recap, grind off the bayonet lug if there is one there, and replace the flash suppressor with a permanently installed muzzle brake, and you're home free.