As per John above, this is the head of one of the bolts you need to remove to detach the ecu from the battery tray:
View attachment 454096
Not the black nut inside the bracket, that is welded on. It's the silver coloured head of the bolt on the outside of the bracket, facing the engine.
I don't know if you can get a spanner on it, might need to be a socket.
You'll need a socket and extension to get the battery tray off.
Unfortunately fiat kept changing the mounting of the ecu, making it hard to cross reference.
Any plastic clips need squeezing and pushing through the holes in the battery tray. Then some bolts and screws to remove.
You're getting there!