John and Doves,
Yes, now you have started something!!
Barry mentioned the chequerboard on the wall. If you count the squares you will see they are 9 high by 17 wide. There seems to be a row at an angle at either end too but the front face shows 9 x 17 = 153!
Also, D is the 4th letter of the alphabet and P is the 16th. Inbetween are 11 letters or inclusive are 13.
Also, perhaps DP means Depart Presently or Permanently.
Keep looking up!
Thanks, Phil and Barry! Looks like there will be a part 2 ......