It is part of the wing tank adapter at the rear brace. This link shows a model wing tank. The front of the silver colored tank is to the left. Look at the darker gray colored top part that the tank attaches to. (That gray part (the pylon) stays attached to the wing of the plane.) Just in front of the rear attachment point is a odd shaped piece with a bulge. This is the part in your blueprint. There is a similar bulged part just behind the front attachment point.
http://firepowerrc.com/ESM-R-pylon-w-tankw.jpg
It is hard to find photos of real planes that show the details, but here is one that shows the pylon with the bulges and also the attachment points further out on the wing for rockets.
https://www.cybermodeler.com/aircraf...c_p-47d_08.jpg
The pylon carrying a bomb instead of a drop tank. (Click on the 3rd picture down in the text section to get a better view.)
http://gettingthestoryright.blogspot.../ie-shima.html