I'm not familiar to US slotted FH for S12, if it made as ordinary FH (> 18,5 mm Dia), not as Original Saiga-12 slotted choke cylinder (18,3 mm Dia) - it is safety to use it on V12, other way it is like to play Russian Roulette, especially with Slugs or Semi/Magnum Buckshot shells - barrel damage, in that case, is not a warranty issue, as well as med aid coverage .
hdskumm wrote above, that US FH has bigger internal Dia than the V12 barrel so there is not any problem to use it, it will work only as a FH not as Cylinder choke+FH, that extend barrel length a little bit.
IMHO, the way