Without looking at the link... The sector is the piece that attaches the moldboard to the A frame. The sector pivots (on the king bolt, which passes through the sector and a frame), thereby angling the plow. The angle cylinders, and trip springs attach to the sector.
I would guess that is a homemade A frame and sector. The reason being that Meyer plows offer (or used to offer) power angle as an option. Without the "half moon" portion of the sector, and without the tab on the A frame, you could not use a pin to hold the angle or straight position of the moldboard.
The tubular A frame is something I have not seen before, even on very old Meyer plows for Jeeps.... Hence I think it is homemade.
If it was mine, I would make a new A frame, and buy a new sector, either that, or buy a beat up 6.5' Meyer plow, and take the sector and A frame from it.
~Chuck