That is a waterproof tube to hold the Tax Stamp paper which must be displayed and of course is changed annually. The current Thai calendar year is 2561 so that is a paper inside simulating year 255X. The tax stamp is usually put inside the car or truck window or it gets stuck it on a bike under a clear plastic sticker. I don't want it stuck on my bike like that so I bought the tube, I haven't gotten the License Plate or Tax Stamp yet but should get them in a few weeks.