A note about 38mm bay design - the tube diameter does not allow for much room to stack components, as you can in 54mm bay designs. The 38mm line introduces a short battery sled which provides a way of holding and retaining a battery in an area next to a radial switch and an altimeter sled. With some of the smaller altimeters, it is possible to do a custom long sled that incorporates a battery retainer instead of requiring a second sled for just the battery. I have already designed and made available a long PerfectFlite StartoLoggerCF sled with a 9 volt battery retainer. On an even smaller note, with a specific 300mah 2S LiPo battery, it is possible to stack a battery and an extra short sled (78mm long) for a compact 38mm bay that fits in a 6 inch long coupler tube.
Also, please make sure you order the correct 38mm coupler sized product. The parts that come in different sizes will have a choice of coupler tube size. There is a thin wall size that fits Blue Tube 2.0 and there is a think wall size that fits MadCow's 38mm coupler tube.
This ring provides three 4-40 threaded inserts for mounting caps and bulkheads which can be for deployment or such things as antenna connections and there are three 6-32 machine screw nuts at 120 degree intervals for mechanically securing the ring inside of a coupler tube if needed. Also, there are three additional threaded inserts for securing switchs and connector plates.