It turns out to be an extendable radome housing an H2X radar. The TBM was a control plane for a TDN/TDR drone equipped with a transponder and a TV camera. In 1943. The controller in the TBM could guide the drone to the target using the radar and then dive it into the target using the TV image. Neither the drone nor the target needed to be within eyesight of the control plane.
Although it worked, the Navy fighting the war in the Pacific would have nothing to do with it. At the insistence of the Navy in Washington, the Special Unit involved was finally allowed a 30-day trial in late 1944 against Japanese targets on and near Bougainville and Rabaul that arguably demonstrated the effectiveness of the concept. It was then summarily withdrawn and disbanded. (It's not clear whether the beyond-visual range capability was utilized.)
For a good summary of the TDR (the very similar TDN wasn't used in combat), see http://evanflys.com/tdr-1
The photo is National Archives 80-G-387191.