Hexagonal spanner for Fidelity Research FR 66S tonearm

this one has a heavy weight with two flat spots for a tool to tighten/loosen

this one has a hex nut

someone could have changed it, I had to buy a nut for one of my friends Micro Seiki 505's. 

You have the arm board loose, take it with you to a great hardware store.

this heavy ring has no flat spots, it has two holes, you temporarily put a rod in the hole and tighten it with the rod. Note, you want a washer below the board, above the heavy ring. Scroll up, the top ring with flat spots also has a hole so it can be tightened/loosened by that method also, in case a tool won't fit.

My prior undisputed favorite arm, very unique magnetically controlled Acos Lustre GST-801 (I got with help choosing and finding from members here).

I like my restored JVC Victor UA-7082 equally, and it has the advantage of being 11-1/8" effective length, so it’s my new favorite. Both have removable headshells.

Elliot, In your first photo, that huge nut with two flat sides is the unique nut for the B60 base.  That's a costly option that facilitates VTA adjustment and also adds mass to the tonearm base. Without the B60 base, the nut is hexagonal and much less massive, and you adjust VTA with set screws as shown in your last photo.

Believe mine is like Elliot's, with the 2 flat sides. Am hoping to find the right tool locally.

So maybe my memory is faulty in that the huge nut with parallel flat surfaces is common to setup both with and without the B60.  But if that is the case, why would your manual refer to a "hexagonal" nut? With the B60 nut, no hexagonal wrench is appropriate.  Just grab it with an adjustable wrench, the kind with two flat surfaces that move together or apart with the spiral shaped adjuster, and Bob’s your uncle. More likely, you have the B60 VTA adjuster there, which would account for the discrepancy between the language of the manual and the nut you are looking at. The B60 adds at least $500 of value to the ensemble, IMO. Maybe more.