Yes they are related to ASME B16.34 Limited Pressure Class. Do a google search for "Class 1690 Pressure rating" and you will find numerous valve manufacturers offering product with these ratings. You should also look at B16.34 section for limited class to understand the differences with the standard class ratings and the specific rules. For example, limited class is restricted to NPS 2-1/2 and smaller, weld and thread end only. Only specific body shapes are allowed. Limited class uses a different equation to establish the pressur/temperature rating for group 1 and 2 materials, which can yield a higher rating than standard class. The benefit is that you can get a less expensive valve with sufficient pressure rating and conformaning to the required code without using the next higher standard class. For example, if your system operating pressure is 1,500 psi, and class 600 max. pressure is 1480 psi, you must either go to class 900 or use the limited option.
