I found a small magnet imbedded in. The airfield (diorama) eliminated the need to add lead. Here is a picture
Balance Your Black Widow Without Lead
Reg
I realize this is almost a year too late, but there is a way to fix the tail sitting problem without disassembling the plane and adding lead. I did an article for it on my website scale modelers handbook where I am doing a build article on the Black Widow.