The easiest thing to stand up on a hot aga plate for 5 minutes would be a silencer.
As the CT say 'there was more than one'.
However holding a rifle barrel or poker on a hot aga plate for 5 minutes is not hard.
Zoso suggested a cattle prodder.
As always. Options for Bamber.
Any metal implement heated to the degree the end was capable of causing a burn would radiate heat and be impossible to hold without inflicting a nasty injury to the hand holding the end.
I'm with Prof Knight they are not burns.
If I was pushed I would say they are marks caused from within eg metal plates, screws or such like which given NB's final resting position ie upper body bent over and spine curved took on that appearance post death.