I don't think it matters. There even appears to be an offset.... looking from the top, one side of the arm is offset or indented a little. However, when installed, the offset sides face the same direction.... either both left or both out.
I believe there are least 2 different lower arms used on our 65-66's, the difference only being the holes. One has an 'O' stamped on the top surface.
FWIW, my '66 has the oblong holes (on top) towards the rear axle, round holes towards frame.