Bob,
Those are datum lines or reference lines if you prefer and are measured from the centerline of the body for lateral holes like on the firewall (see Section 1), and from an imaginary 0 horizontal reference line (CL) measured down from a point below the lowest point on the body (3.85" on the 62, I don't have a 63 assembly manual) and a fore and aft 0 reference line located a specific distance rearward from a point on the top of the radiator support filler (Sheet 14 Sect 11-13 in 62 AIM).Positive numbers are above the horizontal line and aft of the vertical line. Negative numbers are forward of the vertical 0 line and positive aft.
It takes a few steps to get to the hole you want to locate but it works.....kinda like tramming a frame.