I've never had this problem, but I would assume it is seized due to dust/dried salt in the gaps between the extension and housing. With most things that are seized in place, adding a tiny bit of WD40 help to create lubrication 95% of the time works.
I imagine as long as you gently fill the open port hole with cloth - say a tshirt etc (protect the inner workings), and spray the wd40 gently and sparsely around the outside of the extension/housing where it screws in, it should be ok....
I have never had this issue, not have I ever used WD40 on a housing, so maybe wait for some others to answer first.
- tapitt likes this