Well, one has to think laterally, and l think the contraption is a so called 'granny bar'.
Lots of older sailors might remember the taller type of same, to hang on to while one is working at the mast.
Hiscock and Moitessier are both mentioning it in their books.
Foolish is mentioning one similar in his book but his is running parallel with the gunwales on both sides, further forward in 'no mans land' between the mast and the bow.
This one is a low profile one for people who do the mast work on their knees, like me. IMO.
A tube would be a bad choice for a self tacker track. For any track.
All self tacker tracks are bolted down like traveller tracks, which they are, at every four or five inches as the pull of the sail would be huge and the force would bend the tube, and or rip the base out of the deck unless it has a considerable backing plate.
The storm sail, which is usually carried on the cutter stay transfers huge forces upon the sheet in high winds! No tube of this size would withstand those forces.