Why are engraved Zippos so common from the Vietnam war?

by thegianttaco

I saw a post today that showed engraved lighters from Vietnam. I know the lighter was standard issue but how did they get engraved? Where did this tradition start?

Link: http://imgur.com/a/yUfm0

malefiz123

Are you sure Zippos were standard issue? Because I'm rather confident the soldiers bought them privately (even though virtually everyone owned one)