How have you structured your tables. One item per record in Materials and one item per record in Products?
You might need a third Bill of Materials table (like lineItems in an invoice) for listing multiple materials for a given product. A portal to the BOM can then be placed on the Products layout and used to link by Stock_Number to records in the Materials table.
Products::ProductID = BOM::ProductID
Materials::Stock_Number = BOM::Stock_Number
with those relationships, you can select a Stock_Number from a drop down in the BOM portal and see data from materials either be copied via looked up value field options or you can just include the field from the Materials table.
Here's a thread that discusses the two different approaches and why you would use one or the other: