LaserJet II printers are very slow at drawing lines and grids. If you simply want to rule across under a section, it is much faster to do this with line-drawing characters. All TrueType fonts have an ‘emdash’ which will make a continuous line when appropriately reshaped. This is normally mapped to the ‘upstyle’ character in your APL font.

leaf.table.List mat
leaf.table.CharSep '—'    © Check the mapping!