These are standard GKN end caps not suitable for a deep plunge joint.
Please note the MS9K001 cap can only accommodate articulation, not plunge as it is a very shallow cap. The MS9K003 works with joints using +/- 11mm of plunge, MS9N002 works with joints using max plunge of +/- 13mm of plunge and MS9K004 works with joints using max plunge of +/- 13mm of plunge