Agreed that its a pretty perfect marking. Attached is a bit clearer image of the bracket. It is showing uneven wear so I am assuming that it and the frame have worn together after 40 odd years of being loose with the weight of the cab on top.
Wonder if Devcon is strong enough to build up he frame area. I will likey have to rebuild or find the frame braket as one side (not shown) that bolts to the side of the frame is rusted through where they tried welding it to the frame.

http://i1380.photobucket.com/albums/ah168/TcolemanC/Mobile%20Uploads/20150802_202327_zpswpxamtmm.jpg


1954 GMC 1400-9430