There are lowers marked 'Multi' not say 5.56 or .223 - this goes back to what the OP had a question about .... and a lower already marked Multi would not be restricted to just 'one' caliber...this is good for several reasons and more so seeing how ATFTB has wordsmith several other clarification letters most of the time just making things clear as mudYou don't have to buy a multi caliber lower. Once you do a Form 1, you are the manufacturer registering the caliber.
Look - you want me to make your head spin... check this out - SilencerCo is releasing a new compact .45 suppressor and registered in that caliber only - you can change endcaps from .45 to 9mm - on a pistol suppressor that you can remove baffles ... this (again depending how you interpret law and what ATF says is a caliber change .... why you ask well because the endcap retains the baffles and thus by ATF is a caliber change + a silencer part...big NO NO.
Now you may argue well what about a rifle silencer in .30 caliber registered as such but I can change the endcap to 5.56? Well - by ATF that's ok because its not a caliber change of the suppressor (the guts cannot be removed) and the endcap is just that a endcap....not a silencer part...LMAO...your head hurting yet?