First of all, there were no Army regulations authorizing or governing the use of helmet markings. Prior to the D-Day invasion there didnt seem to be much use of helmet markings. Probably due to lessons learned from prior amphibious assaults, the ground commanders wanted a way of quickly recognizing the different units of the combat teams in the confusion of the beachhead. It was usualy the unit commanders that required their men to paint the marking to be used for each branch on there helmets.
As for the 348th, they were under command of the 5th Engineer Special Brigade and most would use the Amphibious Forces patch for the helmet marking. Navy Beach Battalion men attached to the engineer teams had a gray band painted around their helmets to distinguish them from army personnel. Officers were identified by a vertical white stripe on the back of the helemet and non-comms had a hortizontal white stripe on the back of the helmet. Not sure why the white arc was used other than to to designate as Engineer Special Brigrade, The arc was used by engineer, quartermaster & military police attached to the brigades.