So, looking closer up.
What has happened, is that one of the E14 headed bolts (for securing the tensioner backplate) has been used in the bottom idler.
The correct idler bolt (with the smaller E12 headed bolt) has been used on the backplate.
The thread, length, and pitch of these bolts are actually the same, but, due to the larger head on the E14 one, you can see below, it doesn't fit properly into the bore of the idler:
As a result, because it doesn't go into it's recess, the amount of thread protruding from the rear of the idler, is somewhat less:
Not sure what the implications of this could have been - if any
I wonder how effectively the head of the bold was securing the idler, on the basis that, not being in the bore, it doesn't actually have much area to purchase on (on the idler face)
It's an academic question, because I've corrected it all - I'm only asking 'cos I'm curious