There are in fact versions of all KS0108 Compatible GLCDs with 20 pins.
The pins #19 and #20 are actually the LCD Backlight and have nothing to do with the GLCD KS0108 Controller, which is why they are not included in the Proteus models.
If you had examined the link that was provided in the previous post, you would have seen the JHD12864E GLCD pinout which details this fact.