Vintage Technics SL-1300 poltergeist--help!

I can't understand why that "channel" exists when I see nothing that interacts with it.  At first, I thought it's there simply to provide better strength or rigidity but that wouldn't dictate the shape of that...???

Mine began acting up and repair attempts weren’t successful. Being someone who doesn’t love the automatic turntables, I gutted the automatic parts, set it up as a manual, and it’s phenomenal—basically an SL-1200 in functionality. New Ortofon cartridge was a big help too. Sound and function are flawless now for four years.

Is the onset of the malfunction temporally related to when you removed the counterweight?

And yes, I think the channel could be there to enhance structural rigidity.

I forgot to return here (OP is me).  In my strange case and video, 3 people minimum did not notice that the Ortofon headshell had a lift arm that protruded higher than the standard Technics headshell.  In fact--that was the problem, full stop.  I discovered the headshell's arm was dragging on the surface of the dust cover until it progressed far enough the dust cover and headshell no longer had interference with one another.  No problem with the SL-1300 works at all.

I am a moron.

Serves you right for using the dust cover while playing records.