That is a good question. How do they handle silencers with different uppers on an AR? I have seen people that have a silenced 223 and 458 Socom on the same rig.
As far as I can tell (I am no professional) you may have to get a separate stamp for the SBS and SBR and then one for each suppressor.
"I ask, Sir, What is the militia?" "Its the whole of the people. To disarm the people is the best and most effectual way to enslave them" -George Mason Co-Author of the second amendment.