These pieces lock to rails on a saddle and allow a small mini pump to be placed sideways under the saddle instead of attaching it to the bike frame. One of the designs is an adapter piece that can be used to bolt in a rack for pump. There racks that come with pumps are usually designed to fit drinking bottle places on bike frames.
Couple of different versions position the pump differently under the saddle. Finally I ended up using the adapter version, so I was also able to connect and disconnect my Otrlieb saddle bag to its rails at the same time.
NOTE: The adapter piece doesn't have as quite wide spacing as the standard drinking bottle bolts have, but it can be modified as such if needed.
My pump holder still fit it just fine (as they always seem to have some tolerance to allow for different bikes frames that are not made exactly to spec).
I printed from black PLA with 0.8mm sides and 20% infill. That seems just barely strong enough for the job. I assume versions would require higher infill to stay intact.
To put things together, you need two M3 bolts and nuts to tighten top and bottom halves together around the rails on your saddle. Other sizes of bolts and nuts can be used, but M3 nuts will tightly fit in the slots in the design.
If using the adapter version, then two additional bolts and nuts are needed for attaching a pump rack to the adapter. Pumps usually come with those. I just used M4 sized bolts and nuts for that.