Yes halter.
make sure you make note of the color of the buttons, if they are the yellow (apple sauce color) those seem to be better with the swimsuit buyers, I think it means the suit is older?
if the strap comes completely off you can also say it can be a strapless suit. Some are attached only on one side and some unbutton on both sides.
I'd say 50's is about right for that suit.
hope that helps, newbie jumping in here.