(l) Weapons and explosives. Notwith-
standing the provisions of any other
law, rule or regulation, no person while
on postal property may carry firearms,
other dangerous or deadly weapons, or
explosives, either openly or concealed,
or store the same on postal property,
except for official purposes.
Seems to me that the other reg would supercede this...being that it says 'Notwithstanding the provisions of any other law, rule or regulation', if another rule/law permits it, than it's legit.
Standard IANAL disclaimer.