Possible Armored Insignia on FS FB

[ghengus]

I got this helmet recently and have been researching but haven’t found anything definitive. I though USMC armored tax mark at first but I think it would be a broken triangle like all other USMC tax marks, not one piece. It is definitely the type of armored insignia the US Army used on vehicles. I thought maybe the numbers are designations like that of armored vehicles. So 4 represents 4th Armored, but there was no 28th Armored regiment or battalion I could find attached to the 4th. So I am stumped and hoping someone on here can assist. Thanks for the help.