I contacted Bella Barista and they wrote back to me that this part number is the guide for the lower spring on the group head.
I also tried contacting 1st line in the US because they have this part number in stock - but there, it is also shown as the guide for the spring. They said it is indeed the guide for the spring and not the top nut.
So, I’m currently back to square one 😞
I’m pasting here the reply from Bella Barista.
We are at a loss why they say this as this is a guide for a spring that does not wear out.
If you describe to us what your issue is we can help more positively, the most common failure of the flow control is a snapped needle into possibly a snapped jet if you describe your issue or take your flow control off and inspect the needle and jet for damage we can help with sorting this.