Here is an old thread where, about half-way down, I posted pictures of my mare Lisa. Note though that I incorrectly identified some of the marks as being from pin-firing. I have since found that pin firing doesn't leave such large scars, but literally little pin marks. White Flecks
Keep in mind that those pictures were probably within a year of the firing being done. The fur has darkend somewhat over the scar area, but in her case still appears at least grey. My other mare has been fired also, but it doesn't stand out nearly so much.