Boeing likes to stamp parts with their drawing numbers. In the first image, you can make out 2220 and 604. In the second image, it looks like 9 2220 70??. There is an engineering drawing, 69-2220, which has a part labelled 604. Drawing title is Cradle Installation for Extra Fuel Tank, which appears to be located in the bomb bay. The drawing applies to the B-17E/F.