First and foremost, I wouldn't concern yourself with forms at this point. They are the last portion of Access development. After you set your tables, you should move on to Reports and the queries that support them. There's no point making beautiful forms if your just throwing your data down a hole from which you can't effectively retrieve it. Reports and the queries that support them make sure your tables are capturing your data correctly and that you can retrieve it in the manner you need.
With that said, let's explore this:
How did you envision that scanned serial number getting to the correct field? Even in your unnormalized database, at some level a person had to let the computer know that the next piece of data was to be associated with "60cm", "900338" and "SN".
In the normalized world this might be achieve by either direct input for each piece of data--> user selects 60cm from a drop down, 900338 from a drop down and SN from a drop down and then scans;
or via a subform--> user enters 60cm and 90038 on the main form, then every item in the subform inherits that data and the user selects SN and then scans in the data in the subform.
Your tables and their relationships dictate how your forms operate. The way your forms operate does not dictate your tables and their relationships.
With that said, let's explore this:
My struggle comes from how do I associate the scanned serial number to the specific part number being scanned?
...
I.e. Item 4 Scan1 enters scanned serial number data into the field for 60cm-900338-SN.
How did you envision that scanned serial number getting to the correct field? Even in your unnormalized database, at some level a person had to let the computer know that the next piece of data was to be associated with "60cm", "900338" and "SN".
In the normalized world this might be achieve by either direct input for each piece of data--> user selects 60cm from a drop down, 900338 from a drop down and SN from a drop down and then scans;
or via a subform--> user enters 60cm and 90038 on the main form, then every item in the subform inherits that data and the user selects SN and then scans in the data in the subform.
Your tables and their relationships dictate how your forms operate. The way your forms operate does not dictate your tables and their relationships.