It appears that Montana never had official statewide inspections.
This 1967 sticker is an example of many Montana auto dealers and auto repair centers that issued this and similar stickers to show a car had been inspected and was safe.
In many cases when a car was pulled over for defective equipment the owner would get the car repaired and they would place the sticker on the car to evidence it had been repaired.
This sticker attaches to the inside of your car windshield.
For an additional $10.00 buyer can have your antique license plate number printed in leu of the serial number.