On my 2001 Babolat Pure Control, I noticed that there are some numbers on the inside part of the frame that begin right above the upper stripe. The first number is +1, then about an inch above that is +2, then up another inch is +3,5. Across the string grommet from the +3,5 is -3,5. An inch above the -3,5 is -2, then finally an inch above that is -1. Does anyone know what the purpose of those numbers are?