This is operational data, the gross weight is chalked on before each cat launch and is calculated from fuel and weapons on board. This information is passed to Pri Fly who list all aircraft in launch order and this order is passed to the catapault launch room and the shooter. When the plane arrives at the Cat it is the launch officers duty to check the chalked data on the aircraft and dial them into a hand held board (used to be a hand written chalk board)with change able numbers (lit at night), the numbers are about 8" high, the launch officer will dial in the numbers and then hold the board toward the shooter, the shooter will cross reference the weight with the list (if used, often they just rely on the board numbers from the launch officer) and check that the correct amount of cat force is dialed in and then shoot when commanded.
The chalked numbers are the last chance weight check before launch, I've some images somewhere that show this board, quite an interesting little quirk of deck operations.
Best
Michael
Addendum, quick google shows the board man behind the aircraft next to the launch pit, but you get the idea, the weight is dialed in and displayed for all the launch crew to see.
http://upload.wikimedia.org/wikipedia/commons/0/05/US_Navy_021211-N-0012S-001_Two_Flight_Deck_Catapult_Officers_signal_to_launch_a_T-45C_%5Eldquo%2CGoshawk.%5Erdquo%2C_.jpg note, the launch officer also has a list to confer with as well. This is a waist cat and the shooters are at the side of the deck, you can see one in the back ground with arms raised for cat 4, the T-45 is on cat 3 , the launch pit is between the two. On later carriers I believe the lauch pit duals up with the shooter and is an elevating cupola with blast proof windows, but I'd have to check my notes when I get home from work to be sure.
Another shot here
http://upload.wikimedia.org/wikiped...ies_for_launch_aboard_USS_John_F._Kennedy.jpg the lauch pit has the two dials for cats 1 and 2 on the underside of the lid of the pit, facing the lauch office when the lid is raised. The board man is the middle of the three standing green shirts on the right and is holding the board at chest level.
Another showing the exposed launch pit on the waist cats
http://upload.wikimedia.org/wikiped...9-N-1346L-003_USS_Kennedy_-_Hornet_launch.jpg, the board man seems to have gone walkabout, on the rail are the two shooter posistions, the single green shirt (cat3) with one hand rasied has just lowered his right hand to push the launch button, two arms up means the cat is primed and ready to go with the correct steam for the weight. To his left are three green shirts at cat4 shooting station, that cat is not primed, hence no arms raised, they will prime that cat once cat 3 is clear, this is to prevent accidental triggering of cat 4 which will force that aircraft through cat 3 and destroy both aircraft, a safety interlock prevents cat 4 being primed whilst cat 3 is in use.
