The pencil mark grid leak resistor was primarily a home constructor device that worked the same way as the commercial versions but was cheaper. It
was just a line drawn on a piece of cardboard usually with screws, nuts and washers as contacts. Experimenters liked them because you could rub on, or erase off, pencil lead until you got optimum resistance. They were somewhat permanent. As far as the type of line used, you just drew back and forth between the contacts until you got the resistance you wanted. I have a radio with one and it works just fine. Grid leak resistors allowed electrons to leak off the grid to keep the operation of the tube from blocking up.