When you look at the front of it (face of the gauge) the needle is at the center and to can go to the left (negative, or discharge) or to the right (positive, which means it's charging). I believe the 2 terminals on the back correspond with that.
At least they do on mine, and mine charge okay
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'70 107 with k301 engine swap
'71 106 with 38" deck
'70 147
R with factory replacement k321, 42" deck
'61 Original with 38" timed deck
'63 70 "pinkie"
1863 with 54" deck
'46 Farmall H, '50 Farmall Cub
105 x2 (parts)