Toeboard gusset to frame bracket.� I did see that in DL Bensingers listings for the MB/GPW, and I believe it is the same part. For some reason the 3A body parts are not online on anybody's website from what I can tell.
There is also a floor brace that goes between the gusset and the frame, and back underneath the floor. If your gusset is gone it is probably junk as well. I did see it in the online catalogs in the section with the floor hat channels.
BMIC, it might be worth giving one of the major parts places (Walcks, Kaiser Willys, DL Bensinger) a phone call about your needs. None of the usual suspects seem to have a complete tub parts listing online.