Walnut shells, the magic media.
Did they "dye" them afterwards.
I like that because I figure it protects the plastic from the evil ozone UV demons.
Just bought my wife one of these vintage pens for our anniversary.
Classic.
http://www.parker75.addr.com/Reference/Cisele/Crosshatch_grid.htm
The early versions, however, had the lines filled with black enamel so that the grid was accentuated. The process of placing the enamel was actually quite simple. The pen was simply coated with this material. To remove the excess enamel off the sterling silver, the pens was placed in a washing machine-like device with a drum containing a rotating agitator; it was filled with
chips of walnut shells. The action of the chips rubbing against the pens caps and barrels would remove the excess material but, being larger than the line widths, the chips could not touch the enamel within the lines. The process was completed with polishing the pen.