The asbestos-style cassettes are designed with a unique outlet shape, often referred to as a “bell” shape, and are commonly used for sampling for asbestos. They are available in different configurations, including 2-piece and 3-piece designs with a 2″ long spacer ring known as a cowl.
It appears that you are describing different types of conductive cassette housings that are used for air sampling in various applications, including sampling for asbestos. The conductive black polypropylene cassette housings are designed to be leak-free and are made using carbon-filled polypropylene that makes them conductive when grounded properly. The different types of cassette housings vary in their design and are suited for different sampling applications.
The two-piece standard-style housings have a lower surface area compared to three-piece or four-piece housings, and are ideal for closed face sampling applications where minimizing contaminant clinging to the housing walls is beneficial. Three-piece standard-style housings have an additional ring separating the inlet from the filter media, and offer greater flexibility for both open and closed face sampling applications, and are compatible with aluminum cyclones that connect to the spacer ring. Four-piece standard-style housings have two additional rings and allow for a center filter placement to meet certain European standards and for the shoot-through FTIR cassette.