Spinnaker poles are heavily loaded on compression. I'm wondering about a "lockable" design for that (I have seen some with holes and pins).
You sure it's a spinnaker pole and not a whisker pole?
A whisker pole, intended to pole out the jig, will be a lot lighter. (The relative strength difference is quite noticeable on bigger boats - on a smaller boat you might size for the more demanding application and then reuse for both).
If it used to lock (and the mechanism is just broken) perhaps you can convert it to a pin-locked version by drilling a hole through both tubes when extended to the correct length and then putting a long pin through?
I'm fighting my lockable tiller extension, but at least that one does lock - it's just too easy to inadvertently unlock it and losing all steering. My spinnaker pole is home-built from a length of bamboo, which has worked well for me. For years, my "whisker pole" was a paddle, with the blade wedged against the mast and a short bungee looped around the handle, so I could use one of the "hooks" to hook into the jib clew. So, no, I don't have a lot of experience with extendable and lockable poles.
Do you have a spinnaker for your boat? Or were you going to use it as a whisker pole.