Sorry, I should have been clearer. The thickness you have been quoted is the overall thickness of the door including the hollow compartment at the back which houses the bolt mechanism and the locks. What you need is the thickness of the steel plate which makes up the door. That is how doors are graded.
I notice that the bottom plate of the doorframe is flat. That probably indicates that it is a ½″ door. From memory the lighter door had a frame where the bottom was made from angle. Of course designs change and with a second hand door you can’t rely on the frame being original as often the easiest thing to do is to flip the door off, leave the frame in situ and simply fabricate a new frame.
Such doors can represent excellent value for money. If you look at the cost of a strong fire door, door frame, couple of good locks, hinges and hingebolts etc. you are running into quite a lot of money and the level of security offered is not comparable to even the lightest strongroom door.
