Undetermined Viet.Nam Damm Pinback button

Here is a Semi psychedelic Viet.Nam Damm button, 1 3/4 diameter and that is about all I know about it, Anyone know the back story of this one? Most often when Vietnam is broke into Viet Nam it was from c1965 or earlier but I don't know in this case, There was a few "Give A damm" buttons and a THOMAS WILLIAM DAMM who is honored on Panel 9W, Row 126 of the Vietnam Veterans Memorial in Washington, DC. and then we bombed a Dam or two, Or just a statement Viet Nam Damm (in place of Damn)
Any ideas?
P.S. this is what is called a "Day Glow" button and is much brighter more vibrant but they don't scan well it even hard to get a photo that looks right.