There was an option for pilots to add ferry pack under the canopy hood to have some small space for personal things during transfers. My P-51D “Ridge Runner III” seems to had the ferry pack installed on it’s last mission on 18. April 1945. While strafing Kbely airfiled in the Czechoslovakia, Pierce McKennon’s Ridge Runner was hit into the canopy and broken pesrpex pieces wounded pilot’s eye. The canopy was photographed after the successfull landing.

From the photo above it’s evident that the ferry pack is installed. Next photo shows the empty ferry pack installed on the unknown Mustang:

Remaining photos shows my scratchbuilt empty ferry pack in 1:48 scale: