Looks like you figured it out while I was typing this up!
Unlike other databases, Airtable doesn’t use keys.
Airtable only links entire records to each other by their primary field, and those links are performed manually by the user (not automatically by Airtable). You can have more than one link in a table, but it will always be linked to the primary field in the other table.
So if you need your records linked together between Table 1 & Table 2, you would need to make your primary field in Table 2 the Table1_Key. Then, back in Table 1, you would need to make your Table1_Key NOT the primary field. Then, you can convert that field to a linked record field that links to Table 2.