Drove across town to my local Ace, and they had a more extensive selection of hardware vs. Lowes or Tractor Supply.  M8X1.0 worked when fitting the rear bumper bolts into their measuring rack at the store.  I bought $17 worth of bolts eager to replace my crusty bolts.  Alas - they don't thread in the Roxor.
I'm posting this in case any other person has this issue replacing these bolts.  M8X1.0 thread pitch is not the winner.  I also attempted M8X1.25 without success in these 8 holes on the bumper mounting area.
Rather than attempt to buy these sight unseen in BSW thread which will lead to frustration as I don't know the thread pitch, methinks either rust removal, repaint and lube of current hardware or eventual retap and thread is the way to go.