IMPORTANT NOTE: The Silver Spring post office is much smaller than the Bel Air post office which necessitated a modification to the typical GRID designation. Be sure to read the GRID instructions carefully to make sure you are looking in the correct spot.
Grid:
When you enter the post office you will find post office boxes. Due of this very limited selection of boxes: For the purposes of this geocache the Grid refers to a group of boxes separated by a vertical metal strip or a stand alone unit.
The Grid number is determined by the number of the P.O. Box in the upper RIGHT-hand corner of the Grid. Find the P.O. Box number that matches the Grid number and you are standing in front of the Grid you need to use.
A Grid has rows of boxes that run from left to right and columns that run from top to bottom. The P.O. Boxes at the bottom of the grid are usually bigger than the boxes at the top. The rows will normally line up across the entire grid, but the columns will not. Typically, there will be more columns on the top of the Grid and less along the bottom of the Grid.
Row:
Starting at the upper left corner of the Grid and counting from top to bottom, the top Row is 1 the next Row down is 2 and continuing on to the bottom of the Grid.
Column:
Counting from left to right, the leftmost box is Column 1 and the next box from the left is Column 2 and continuing on to the right of the Grid. Warning: Row 1 will probably have more Columns than Row 9, so be sure to be counting across the correct row.
All that is left is a little addition with the P.O. Box numbers and you will have the cache coordinates.
To find the cache location, use the (Grid:Row:Column) format.
N 40 03.(G351:R6:C3) + (G231:R3:C6) + (G271:R4:C2) W 076 26.(G51:R8:C2) + (G111:R9:C4)
The P.O. BOX DIGITS you found should add up to 51. The final location is within .1 mile away.
The P.O. Box lobby is open to the public 24 hours.