There are just a couple of acronyms that are used in the descriptions, so any references to BT mean Body Tube and CT mean Coupler Tube. 54mm is used as a general term for the basic size rather than an absolute measurement. The parts here are sized for standard blue tube 2.0 or any similar sized product. If you have any questions or a special size requirement, please contact SMT Designs on the Contact Us page. I will adjust diameters of parts for you given your measurements for no charge other than the normal product price (only caveat - non-returnable unless I made them wrong).
The photo shown is an assembly of individual parts for a medium sized dual deploy altimeter bay. It is meant for a larger rocket than 54mm due to the BT end cap on the left side of the bay. A completed bay fits in a Coupler Tube (CT) and is held in a Body Tube (BT) by a standard motor retainer. A completed assembly includes an arming switch accessible through the rocket's wall from the outside, a battery tray, and a sled with a connector for field changes. O-rings can be used on each end to seal the bay from ejection gases if needed.
The 54mm CT Float bulkhead is a do it yourself part. It has SMT Designs' standard ring pattern on one side (hole depth is 5mm) so it can be attached to any of the other SMT Designs parts that have the same pattern.
The 54mm BT/CT end cap is a do it yourself part. It has SMT Designs' standard ring pattern on one side (hole depth is 5mm), so it can be attached to any of the other SMT Designs parts that have the same pattern.
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.