Don't see why you are using two separate files, one for the Master List and one for the Inventory List when you can put both tables in the same file and then simplify your design.
A description of the relationships that you have set up will be helpful, but there are two basic approaches you can consider. Each method is the 'best' choice in different circumstances.
Data from the related Master LIst table can simply be displayed on the Inventory LIst table by placing fields from the Master LIst table on your Inventory LIst table.
Or datat can be copied from the Master LIst into fields in the Inventory List via auto-enter field options.
Both require the same basic relationship between the two tables and this will work regardless of whether the tables are in the same file or separate files so long as you set up an external data source reference and relationship.
Here's a link to a thread that discusses the two basic options I have briefly described in greater detail: Auto Fill