Front Stub Frame body mounts

Have a question regarding the front stub frame body mounts on a 68 Fury. I currently have the frame removed and while cleaning the rear mounts noticed 4 holes 2 front ,2rear where the mount meets the sill. This is the rear mounts not front shimmed mounts. It appears there is threaded holes ? Also appears to be welded? See attached pic ,any help thanks. None of the thread topics seemed to include chassis so was unsure where exactly to post?

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Author: Phoenix440