I put an older distributor into my '77 and it's got that gizmo. I put it at that midpoint mark for no reason other than to be sure it wouldn't move without me knowing it. I set my timing at or about 9*, have never had any detonation probably because one of the first things I ever did was desmog.
Doesn't the book indicate that each mark equals 10* ?
I don't use the adjuster on the '77 but my '66 has it too along with the handle for loosening the clamp so I tried it on that truck. It worked, moved the timing more like 10* for a half turn which made me wonder if that one might be broken some way, but again I'm not sure if it's supposed to have the sensible midpoint zero.
Yours sure is spiffy. Mine has "patina" SOR still stocks the little plastic cover for about $10.