I've been around long enough to remember the days when cars and trucks got brand new license plates every year. That was pretty cool, and I kind of miss it ... not only did the colors change every year, but after a decade or so you were able to fill a garage wall with your very own vintage license plate display. Nowadays, Montana issues new plates only every five years, which just isn't the same.
The tradition of annual license plates in Montana lasted until 1976, but there were exceptions even before that. In an attempt to conserve metal near the end of both World War II and the Korean War, updated license plates were put on hold for a time, and window stickers were used instead. Here's a replica of the sticker used in 1947, which is pretty cool in itself: