Key guard thing = fallboard
426000 is most likely the serial number. Steinway grand models usually go by a single letter: S, M, L, O, A, B, C, D.
EDIT: See Richard's post below for special Steinway instructions.
Simply remove the fallboard. Try to maneuver it into a semi open/closed position. In a certain position it will be easiest to just pull out. Careful not to scratch the case with the hinges while removing the fallboard. Fish out the pencil careful not to get anything caught in the action parts. There may even be a 426000 stamped on a hidden part of the fallboard if you look at it closely. Replace the fallboard.
This procedure above works with most grands. Some detective work may be needed to find out the best way to remove the fallboard on a particular piano, and place it back. It is such a common occurrence to drop pencils in there. If you own a grand, its good to become familiar with removing the fallboard.