This is a single port, pneumatic cylinder or actuator.  The outside diameter is 17mm and guessing the wall thickness of the stainless steel as 1mm, this makes the diameter 15mm.  The 5mm rod will extend 100mm and automatically returns with an internal spring. Pressurised air needs to entry through the end port and exit through the same port when the rod will return to the rest position. The port at the back is 5mm metric ISO coarse.  I have tried this with a cap head screw.  Adapters are available to screw in and take the hard plastic push in tube.  The front collar is 10mm in diameter.  Working in old units, the piston head is just over 0.25 square inches and with 100psi (7 kgf/cm2) would give 25 pounds of push.  It was bought for teaching, has seen little use, age and moving means it has to go.