This looks like what I have on my Catalina 27-it is a whisker pole, used to pole out the jib/jenny when sailing wing and wing downwind. (You can also use it to pole out the jib/genny when on a broad reach and there's not enough wind to keep the sail out there.)
The hook in the first picture hooks to the ring on the mast, and I run the jibsheet on the outside of the sheave in your second picture. The pressure of the jibsheet holds the outer end of the whisker pole up against the clew of the jib. I think the hook on the outer end just keeps the sheet from jumping off the sheave, and makes it easier to hook the pole to the sheet when the sheet's out there a ways.
And no topping lift is needed-the pole's light enough for the jib to support it.
And this pole wouldn't serve as a spinaker pole on my boat-it's too light. My spinaker pole is fixed in length, ~10-12' long, ~4" in diameter, and requires the spinaker topping lift to support its weight and the (darn, blanking out on what it's called), line coming down from the middle of the pole to the foredeck to keep it from rising up.