hmt Rajat Automatic (Citizen Licensed Caliber) 21 Jewels
Most of the hmt Watches
come with this kind of very inexpensive clasps. The same mechanism that requires no tools to adjust is still popular today for many lower tier watches. Even the Casio is often using these clasps on their lower end watches. You simply use a ballpen or a paper clip and lift the part of the clasp with the little arrow. Then you slide the clasp to the position fitting your wrist.