Mitutoyo is unquestionably the leader in innovative digital gages, including these digital "drop" indicators. Being innovative means they also change the designs quite regularly although sometimes the model numbers are changed on purpose to confuse the Chinese who are always ready to produce imitations.
When buying Mitutoyo, make sure you get the latest models and avoid any old models or suspiciously inexpensive models.
Also, buy from a reputable Mitutoyo dealer, of which there are many. If you aren't sure, you can always call Mitutoyo to find one in your area.
Amazon is actually one of the Mitutoyo dealers and provides authentic gages. If you will click "check price" below, you will be presented with Amazon's information on these gages and you are welcome to place an order. They have very good return policies if something goes wrong and you can often get free shipping, as well as the best discounts.