We are measuring the inside diameter of the bezel in order to determine which size crystal to buy. The diameter is 38 mm using a simple steel rule. A dial or digital caliper may give you a more accurate reading of course. This is not the outside diameter of the bezel, nor is it the inside diameter. This measures the top of the ledge on which the crystal will sit once it's pressed in place with a crystal press. You will want to order a crystal that is at least one milimeter larger. In this case, you will want to order the 39 mm diameter crystal. When pressed in place, the crystal will form a dome and will stay put inside the groove on the bezel.
Not all indicators or calipers use flat crystals in this way. Be sure to read the detailed information shown on page 233.