I understand your issue and why it’s not working, but i can’t realize your goal in common. What’s the purpose of Diameter table and why do you choose autonumber for primary field of Attributes?
In short, linked field connects one table to another table (not field), and when you set links (which connecting records to each other), you will see the “record name”, which is a value of primary field. So, when you connect Junction to Attributes, you see its records as a set of numbers(from autonumber). Of course, you can act from “other side” - go to the Attributes table, connect Diameters, copy-pasting diameter values column into that linked field, add lookup field (Junction links from Diameter table), copypaste them into linked field with Junction.
But for now, such base design looks a bit… “inconvenient”? If you are planning to add more links in future, searching from Juction table to diameters in Attribute, maybe you should make this column a primry field?