Vintage Technics SL-1300 poltergeist--help!

Thanks!  He really never addresses weird channel (or whatever on that plastic gear/cog wheel/whatever...).  I need to go back further than where it started playing for me.  I'd like to see what interacts with that channel, if anything.  There must be something--else why have it molded in like that?  In other words, when he is ready to button it up there appears to be nothing that is going to touch it except on the side that becomes hidden during reassembly.

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.